DRW: Add new Draw Manager OpenGL Context.

This separate context allows two things:
- It allows viewports in multi-windows configuration.
- F12 render can use this context in a separate thread and do a non-blocking render.

The downside is that the context cannot be used while rendering so a request to refresh a viewport will lock the UI. This is something that will be adressed in the future.

Under the hood what does that mean:
- Not adding more mess with VAOs management in gawain.
- Doing depth only draw for operators / selection needs to be done in an offscreen buffer.
- The 3D cursor "autodis" operator is still reading the backbuffer so we need to copy the result to it.
- All FBOs needed by the drawmanager must to be created/destroyed with its context active.
- We cannot use batches created for UI in the DRW context and vice-versa. There is a clear separation of resources that enables the use of safe multi-threading.

1 files changed, 4 insertions, 0 deletions

diff --git a/source/blender/editors/space_view3d/view3d_draw.c b/source/blender/editors/space_view3d/view3d_draw.c
index 6e3a0883489..68996cdf906 100644
--- a/source/blender/editors/space_view3d/view3d_draw.c
+++ b/source/blender/editors/space_view3d/view3d_draw.c
@@ -2131,11 +2131,13 @@ ImBuf *ED_view3d_draw_offscreen_imbuf(
 	}
 
 	const bool own_ofs = (ofs == NULL);
+	DRW_opengl_context_enable();
 
 	if (own_ofs) {
 		/* bind */
 		ofs = GPU_offscreen_create(sizex, sizey, use_full_sample ? 0 : samples, true, false, err_out);
 		if (ofs == NULL) {
+			DRW_opengl_context_disable();
 			return NULL;
 		}
 	}
@@ -2267,6 +2269,8 @@ ImBuf *ED_view3d_draw_offscreen_imbuf(
 		GPU_offscreen_free(ofs);
 	}
 
+	DRW_opengl_context_disable();
+
 	if (ibuf->rect_float && ibuf->rect)
 		IMB_rect_from_float(ibuf);