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_vertex_buffer.cc

@@ -29,6 +29,10 @@ namespace blender::gpu {
 
 void GLVertBuf::acquire_data()
 {
+  if (usage_ == GPU_USAGE_DEVICE_ONLY) {
+    return;
+  }
+
   /* Discard previous data if any. */
   MEM_SAFE_FREE(data);
   data = (uchar *)MEM_mallocN(sizeof(uchar) * this->size_alloc_get(), __func__);
@@ -36,6 +40,10 @@ void GLVertBuf::acquire_data()
 
 void GLVertBuf::resize_data()
 {
+  if (usage_ == GPU_USAGE_DEVICE_ONLY) {
+    return;
+  }
+
   data = (uchar *)MEM_reallocN(data, sizeof(uchar) * this->size_alloc_get());
 }
 
@@ -94,8 +102,10 @@ void GLVertBuf::bind()
     vbo_size_ = this->size_used_get();
     /* Orphan the vbo to avoid sync then upload data. */
     glBufferData(GL_ARRAY_BUFFER, vbo_size_, nullptr, to_gl(usage_));
-    glBufferSubData(GL_ARRAY_BUFFER, 0, vbo_size_, data);
-
+    /* Do not transfer data from host to device when buffer is device only. */
+    if (usage_ != GPU_USAGE_DEVICE_ONLY) {
+      glBufferSubData(GL_ARRAY_BUFFER, 0, vbo_size_, data);
+    }
     memory_usage += vbo_size_;
 
     if (usage_ == GPU_USAGE_STATIC) {
@@ -106,6 +116,37 @@ void GLVertBuf::bind()
   }
 }
 
+void GLVertBuf::bind_as_ssbo(uint binding)
+{
+  bind();
+  BLI_assert(vbo_id_ != 0);
+  glBindBufferBase(GL_SHADER_STORAGE_BUFFER, binding, vbo_id_);
+}
+
+const void *GLVertBuf::read() const
+{
+  BLI_assert(is_active());
+  void *result = glMapBuffer(GL_ARRAY_BUFFER, GL_READ_ONLY);
+  return result;
+}
+
+void *GLVertBuf::unmap(const void *mapped_data) const
+{
+  void *result = MEM_mallocN(vbo_size_, __func__);
+  memcpy(result, mapped_data, vbo_size_);
+  return result;
+}
+
+bool GLVertBuf::is_active() const
+{
+  if (!vbo_id_) {
+    return false;
+  }
+  int active_vbo_id = 0;
+  glGetIntegerv(GL_ARRAY_BUFFER_BINDING, &active_vbo_id);
+  return vbo_id_ == active_vbo_id;
+}
+
 void GLVertBuf::update_sub(uint start, uint len, void *data)
 {
   glBufferSubData(GL_ARRAY_BUFFER, start, len, data);