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_index_buffer.hh

@@ -34,6 +34,7 @@ namespace blender::gpu {
 class GLIndexBuf : public IndexBuf {
   friend class GLBatch;
   friend class GLDrawList;
+  friend class GLShader; /* For compute shaders. */
 
  private:
   GLuint ibo_id_ = 0;
@@ -42,6 +43,9 @@ class GLIndexBuf : public IndexBuf {
   ~GLIndexBuf();
 
   void bind(void);
+  void bind_as_ssbo(uint binding) override;
+
+  const uint32_t *read() const override;
 
   void *offset_ptr(uint additional_vertex_offset) const
   {
@@ -57,6 +61,9 @@ class GLIndexBuf : public IndexBuf {
     return (index_type_ == GPU_INDEX_U16) ? 0xFFFFu : 0xFFFFFFFFu;
   }
 
+ private:
+  bool is_active() const;
+
   MEM_CXX_CLASS_ALLOC_FUNCS("GLIndexBuf")
 };