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, 12 insertions, 0 deletions

diff --git a/source/blender/gpu/shaders/gpu_shader_image_depth_copy_frag.glsl b/source/blender/gpu/shaders/gpu_shader_image_depth_copy_frag.glsl
new file mode 100644
index 00000000000..10f4dfd5a87
--- /dev/null
+++ b/source/blender/gpu/shaders/gpu_shader_image_depth_copy_frag.glsl
@@ -0,0 +1,12 @@
+
+in vec2 texCoord_interp;
+out vec4 fragColor;
+
+uniform sampler2D image;
+
+void main()
+{
+	float depth = texture(image, texCoord_interp).r;
+	fragColor = vec4(depth);
+	gl_FragDepth = depth;
+}