GPU: Compute Pipeline.

With the compute pipeline calculation can be offloaded to the GPU.
This patch only adds the framework for compute. So no changes for users at
this moment.

NOTE: As this is an OpenGL4.3 feature it must always have a fallback.

Use `GPU_compute_shader_support` to check if compute pipeline can be used.
Check `gpu_shader_compute*` test cases for usage.

This patch also adds support for shader storage buffer objects and device only
vertex/index buffers.

An alternative that had been discussed was adding this to the `GPUBatch`, this
was eventually not chosen as it would lead to more code when used as part of a
shading group. The idea is that we add an `eDRWCommandType` in the near
future.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D10913

source/blender/gpu/opengl/gl_texture.cc

@@ -368,7 +368,7 @@ void GLTexture::copy_to(Texture *dst_)
 void *GLTexture::read(int mip, eGPUDataFormat type)
 {
   BLI_assert(!(format_flag_ & GPU_FORMAT_COMPRESSED));
-  BLI_assert(mip <= mipmaps_);
+  BLI_assert(mip <= mipmaps_ || mip == 0);
   BLI_assert(validate_data_format(format_, type));
 
   /* NOTE: mip_size_get() won't override any dimension that is equal to 0. */