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Apparently, glBindAttribLocation() does not work as planned with AMD GPUs. Therefore, all Shaders yielded black images. Replaced all attribute locations with hard-coded ones in the vertex shader and removed corresponding convenience function from cgt::Shader.

refs #382

* cleaned up cgt::Texture's constructors: only two left - to create an empty texture or a non-empty one
* cgt::Texture will no longer hold a copy of the image data in local memory
* removed a lot of redundant/confusing methods
* no longer needed to call uploadTexture() even though you don't want to upload sth.
* Adapted all known code to the new interface
* Removed cgt::TextureReaderDevil

refs #613

refs #386

refs #386

C++ implicit conversion rules made ShaderManager::loadSeparate() ambiguous in some cases. So I decided to cut loose ends and completely refactored loading with standard version into ShaderManager::load() and loading with custom version into ShaderManager::loadWithCustomGlslVersion().
ShaderManager::loadSeparate() is no longer available!

Fixed GlReduction, now working reliably for 2D and for 3D images. Added test pipeline GlReduction test.

* Moving to Apache 2.0 license
* Updated AUTHORS.txt

* Updated DepthDarkening to use GlReduction to compute min/max depth

* OpenGl based reduction in core tools
* SimilarityMeasure processor stub