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/intern/gpu_shader_interface.hh

@@ -60,6 +60,7 @@ class ShaderInterface {
   uint attr_len_ = 0;
   uint ubo_len_ = 0;
   uint uniform_len_ = 0;
+  uint ssbo_len_ = 0;
   /** Enabled bind-points that needs to be fed with data. */
   uint16_t enabled_attr_mask_ = 0;
   uint16_t enabled_ubo_mask_ = 0;
@@ -99,6 +100,11 @@ class ShaderInterface {
     return input_lookup(inputs_ + attr_len_ + ubo_len_, uniform_len_, binding);
   }
 
+  inline const ShaderInput *ssbo_get(const char *name) const
+  {
+    return input_lookup(inputs_ + attr_len_ + ubo_len_ + uniform_len_, ssbo_len_, name);
+  }
+
   inline const char *input_name_get(const ShaderInput *input) const
   {
     return name_buffer_ + input->name_offset;