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.

source/blender/gpu/intern/gpu_shader.c

@@ -77,6 +77,7 @@ extern char datatoc_gpu_shader_image_interlace_frag_glsl[];
 extern char datatoc_gpu_shader_image_mask_uniform_color_frag_glsl[];
 extern char datatoc_gpu_shader_image_modulate_alpha_frag_glsl[];
 extern char datatoc_gpu_shader_image_depth_linear_frag_glsl[];
+extern char datatoc_gpu_shader_image_depth_copy_frag_glsl[];
 extern char datatoc_gpu_shader_3D_vert_glsl[];
 extern char datatoc_gpu_shader_3D_normal_vert_glsl[];
 extern char datatoc_gpu_shader_3D_flat_color_vert_glsl[];
@@ -712,6 +713,8 @@ GPUShader *GPU_shader_get_builtin_shader(GPUBuiltinShader shader)
 		                                         datatoc_gpu_shader_image_modulate_alpha_frag_glsl },
 		[GPU_SHADER_3D_IMAGE_DEPTH] = { datatoc_gpu_shader_3D_image_vert_glsl,
 		                                datatoc_gpu_shader_image_depth_linear_frag_glsl },
+		[GPU_SHADER_3D_IMAGE_DEPTH_COPY] = { datatoc_gpu_shader_3D_image_vert_glsl,
+		                                     datatoc_gpu_shader_image_depth_copy_frag_glsl },
 
 		[GPU_SHADER_2D_IMAGE_INTERLACE] = { datatoc_gpu_shader_2D_image_vert_glsl,
 		                                    datatoc_gpu_shader_image_interlace_frag_glsl },