Also adds a few things to GPUShader for easily create shaders.
Heavy usage of macros to compose the createInfo and avoid
duplications and copy paste bugs.
This makes the link between the shader request functions
(in workbench_shader.cc) and the actual createInfo a bit
obscure since the names are composed and not searchable.
Reviewed By: jbakker
Differential Revision: https://developer.blender.org/D13910
This patch is (almost) a complete rewrite of workbench engine.
The features remain unchanged but the code quality is greatly improved.
Hair shading is brighter but also more correct.
This also introduce the concept of `DRWShaderLibrary` to make a simple
include system inside the GLSL files.
Differential Revision: https://developer.blender.org/D7060
