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
5b299e5999