Fix T73931: Stereo Viewport Color Management

Stereoscopic viewport didn't support Color Manangement due recent
changes in the color management pipeline. In order to solve the issue we
will migrate the strereo rendering into the GPUViewport. This will share
some textures and reduce required GPU memory.

Reviewed By: fclem, dfelinto

Differential Revision: https://developer.blender.org/D6922

source/blender/gpu/intern/gpu_state.c

@@ -218,6 +218,7 @@ typedef struct {
   uint is_sample_alpha_to_coverage : 1;
   uint is_scissor_test : 1;
   uint is_stencil_test : 1;
+  uint is_framebuffer_srgb : 1;
 
   bool is_clip_plane[6];
 
@@ -294,6 +295,7 @@ void gpuPushAttr(eGPUAttrMask mask)
   if ((mask & GPU_VIEWPORT_BIT) != 0) {
     glGetDoublev(GL_DEPTH_RANGE, (GLdouble *)&Attr.near_far);
     glGetIntegerv(GL_VIEWPORT, (GLint *)&Attr.viewport);
+    Attr.is_framebuffer_srgb = glIsEnabled(GL_FRAMEBUFFER_SRGB);
   }
 
   if ((mask & GPU_BLEND_BIT) != 0) {
@@ -352,6 +354,7 @@ void gpuPopAttr(void)
   if ((mask & GPU_VIEWPORT_BIT) != 0) {
     glViewport(Attr.viewport[0], Attr.viewport[1], Attr.viewport[2], Attr.viewport[3]);
     glDepthRange(Attr.near_far[0], Attr.near_far[1]);
+    restore_mask(GL_FRAMEBUFFER_SRGB, Attr.is_framebuffer_srgb);
   }
 
   if ((mask & GPU_SCISSOR_BIT) != 0) {