GPUFrameBuffer: GL backend isolation

This is related to the Vulkan port T68990.

This is a full cleanup of the Framebuffer module and a separation
of OpenGL related functions.

There is some changes with how the default framebuffers are handled.
Now the default framebuffers are individually wrapped inside special
GLFrameBuffers. This make it easier to keep track of the currently bound
framebuffer state and have some specificity for operations on these
framebuffers.

Another change is dropping the optimisation of only configuring the
changed attachements during framebuffers update. This does not give
any benefits and add some complexity to the code. This might be brought
back if it has a performance impact on some systems.

This also adds support for naming framebuffers but it is currently not
used.

source/blender/gpu/intern/gpu_texture_private.hh

@@ -0,0 +1,53 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2020 Blender Foundation.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup gpu
+ */
+
+#pragma once
+
+#include "BLI_assert.h"
+
+namespace blender {
+namespace gpu {
+
+class Texture {
+ public:
+  /** TODO(fclem): make it a non-static function. */
+  static GPUAttachmentType attachment_type(GPUTexture *tex, int slot)
+  {
+    switch (GPU_texture_format(tex)) {
+      case GPU_DEPTH_COMPONENT32F:
+      case GPU_DEPTH_COMPONENT24:
+      case GPU_DEPTH_COMPONENT16:
+        BLI_assert(slot == 0);
+        return GPU_FB_DEPTH_ATTACHMENT;
+      case GPU_DEPTH24_STENCIL8:
+      case GPU_DEPTH32F_STENCIL8:
+        BLI_assert(slot == 0);
+        return GPU_FB_DEPTH_STENCIL_ATTACHMENT;
+      default:
+        return static_cast<GPUAttachmentType>(GPU_FB_COLOR_ATTACHMENT0 + slot);
+    }
+  }
+};
+
+}  // namespace gpu
+}  // namespace blender