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DrawEngine: Shader Test Suite

A test case that compiles all the GLSL shaders for workbench, gpencil, overlay and some
of eevee. Compilation is still platform dependent, but when run on a test-farm
with different hardware we will be able to detect GLSL compilation
errors early on.

The test will be compiled when `WITH_GTEST` and `WITH_OPENGL_DRAW_TESTS`
are On.

For eevee only the shaders inside eevee_shaders.c are included. EEVEE has some shaders
located inside the submodule. They aren't accessible to the outside and aren't added
to the test case. We should see how we want to add them. For the test cases it is better
to move them to eevee_shaders.c, but for eevee perspective it is better to keep them in
the submodule. Keeping them in the submodule could lead to situations that is harder to test.
as the shader could already have been initialized.

Reviewed By: Cl�ment Foucault

Differential Revision: https://developer.blender.org/D8667