Campvis nx

See merge request !7

- Lots of missing override definitions
- some virtual overloaded function hiding warnings
- class initializer list order

Tested on MacBook late 13 with NVIDIA GeForce GT 750M on High Sierra

- Fixed bug with worldToTexture matrix
- default texture clamping is now CLAMP_TO_BORDER

* added buildClang to .gitignore
* Minimum required glsl version is now 430 by default (might be possible to downgrade again by checking on a per-module basis)
* More of the base modules now register their processors into the factory.
* Minor bugfixes in optimizedraycaster (inverted gradient dir) and glimageresampler (renamed variable)

* ambient occlusion refactored as a decorator
* pre-integrated raycaster and processor to pre-integrate TF

Somehow I messed up the cherrypick/merge history.
Stereo module provides basic processors for a compositor and a camera
generator to generate stereo images.

(cherry picked from commit ad722d98127144e0ff34523f210247a92f8dbb20)

(cherry picked from commit 00cdb42948e45681b5cde12629f6970b906b0eec)

(cherry picked from commit b4033e899e318707a732203584c6aee0d9c178c8)

# Conflicts:
#	modules/stereo/processors/stereocameragenerator.cpp

Raycaster is based on the technique described in "Local Ambient Occlusion
in Direct Volume Rendering" by F. Hernell, P. Ljung, A. Ynnermann in IEEE
TVCG 2010.
Implementation does not allow for precomputed LAO volumes but instead
estimates the LAO term per sample.

This reverts commit e7e55015.

# Conflicts:
#	core/glsl/tools/shading.frag

* gradient computation now properly accounts for anisotropic voxels
* Shaders that fail to compile at startup result in an error on the console, with the option to retry computation (after editing the shader) instead of crashing campvis completely

* OpenGL core context is instantiated and enforced, for better forward compat
* Fixed DLL deployment to RelWithDebInfo and MinSizeRel configurations
* Bumped GLEW to 2.0.0
* Fixed wrong shading in various raycasters (normal was wrong sign)

Squashed commit of the following:

commit 21b33709371459552aff972124e9da4ae0077cf4
Author: Jakob Weiss <jakob.weiss@tum.de>
Date:   Mon Feb 6 15:21:12 2017 +0100

    Fixed number of texture unit query

commit 6592f366c7f24822a8681401cf5a415a622e5f6e
Author: Jakob Weiss <jakob.weiss@tum.de>
Date:   Mon Feb 6 14:27:36 2017 +0100

    Fixed core profile requirements

    * now using Framebuffer ARB version instead of EXT
    * fixed extension handling and windows version recognition
    * Removed pixel transfer functions usage and some validity checks for vis pipelines

commit 36c82733434e63309d092bf85f78f471b56bebfb
Author: Jakob Weiss <jakob.weiss@tum.de>
Date:   Mon Feb 6 13:50:08 2017 +0100

    Bumped glew to 2.0.0

commit 4a4a1d51756aad0e19aef9e723e1dfe7d07a797d
Author: Jakob Weiss <jakob.weiss@tum.de>
Date:   Fri Feb 3 16:26:07 2017 +0100

    First Progress - WIP

# Conflicts:
#	ext/cgt/texturemanager.cpp

For some reason, devIL was completely checked in with campvis-public, so we stick to checking in the (customized) windows binaries for now

Stereo module, extended DevilImageReader with builtin filters, minor fix to PredicateDemoSmallHeart pipeline

Attempted colored shadows, but this does not give correct results yet.

* Based on DevIL 1.8.0, built with libjpeg 9.1, libpng 1.6.69, libsquish 1.15, libtiff 5.2.5, openexr 2.2.0, zlib 1.2.11
* Manual patch fixes problem with paletted BMPs (see https://github.com/DentonW/DevIL/issues/56)
* DevIL only built for win64, win32 is considered deprecated as of right now :)

* Light sources now have an additional camera input. If this is set, the light source position is relative to the camera coordinate system
* ImageMappingInformation and Texture3D now supply an additional pair of matrices: textureToWorldMatrixInvTransp to transform normals between world and texture space

* glImageResampler now sets the mapping information correctly
* ImageData now asserts that an associated imagemappinginformation has the
  same dimensionality

* texturemanager documentation and parameter fix
* imagerepresetation conversion fix for channel number mismatch

TextureManager now uses a configurable lifetime for the lifetime of a cache object. Upon every garbage collection event, the counter is incremented for each object in the cache and if a counter exceeds TextureManager::garbageLifetime, the texture id is marked for deletion.
Deletion now happens in a separate thread, as a job enqueued to the OpenGLJobProcessor, in order to properly acquire a valid opengl context.

* AbstractPipeline now does explicit GPU timing, implying a Flush of the OpenGL pieline after every processor
* OptionProperty has an extended constructor directly using initializer lists of GenericOption<>'s
* ImageMappingInformation has somehow extended / more complete interface allowing copies and default construction

* Added .editorconfig
* cgt::Texture::downloadTextureToBuffer now does a glFlush() before downloading
* ColorPropertyWidget now supports transparent colors properly

* minor whitespace fix in mainwindow.cpp

Note: The googletest directory included has been modified from the original release: The CMakeLists.txt has been adapted and gmock and some other unnecessary folders have been removed.