Support seperated video overlays with weston

This is related to the issue, #9

When using "backend-drm" and "renderer-gl" in the "main" branch, the OSD image cannot be ARGB8888 format. With the following modification, it can be ARGB8888 format for kms output. Is there any reason not to use alpha pixel for drm?

diff --git a/libweston/backend-drm/drm-gbm.c b/libweston/backend-drm/drm-gbm.c
index 3db14624..6f8f92d8 100644
--- a/libweston/backend-drm/drm-gbm.c
+++ b/libweston/backend-drm/drm-gbm.c
@@ -294,7 +294,7 @@ drm_output_render_gl(struct drm_output_state *state, pixman_region32_t *damage)
        }

        /* The renderer always produces an opaque image. */
-       ret = drm_fb_get_from_bo(bo, b, true, BUFFER_GBM_SURFACE);
+       ret = drm_fb_get_from_bo(bo, b, false, BUFFER_GBM_SURFACE);
        if (!ret) {
                weston_log("failed to get drm_fb for bo\n");
                gbm_surface_release_buffer(output->gbm_surface, bo);