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_framebuffer.c

@@ -943,6 +943,7 @@ void GPU_offscreen_bind(GPUOffScreen *ofs, bool save)
   glDisable(GL_SCISSOR_TEST);
   GPUFrameBuffer *ofs_fb = gpu_offscreen_fb_get(ofs);
   GPU_framebuffer_bind(ofs_fb);
+  glDisable(GL_FRAMEBUFFER_SRGB);
 }
 
 void GPU_offscreen_unbind(GPUOffScreen *UNUSED(ofs), bool restore)