drm: Get renderer buffer size from drm_fb

The renderer buffer size is usually the same size as the current mode,
so we were taking the dimensions from the currently-set mode. However,
using current_mode is quite confusing in places when it comes to scale,
and it also hampers our ability to do mode switches, as well as to
introduce a future option which will let the renderer use a smaller
buffer than the output and display scaled.

Simply take the dimensions of the renderer's output buffer from the
buffer itself.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/backend-drm/drm.c

@@ -391,13 +391,13 @@ drm_output_render(struct drm_output_state *state, pixman_region32_t *damage)
 
 	scanout_state->src_x = 0;
 	scanout_state->src_y = 0;
-	scanout_state->src_w = output->base.current_mode->width << 16;
+	scanout_state->src_w = fb->width << 16;
-	scanout_state->src_h = output->base.current_mode->height << 16;
+	scanout_state->src_h = fb->height << 16;
 
 	scanout_state->dest_x = 0;
 	scanout_state->dest_y = 0;
-	scanout_state->dest_w = scanout_state->src_w >> 16;
+	scanout_state->dest_w = output->base.current_mode->width;
-	scanout_state->dest_h = scanout_state->src_h >> 16;
+	scanout_state->dest_h = output->base.current_mode->height;
 
 	pixman_region32_copy(&scanout_state->damage, damage);
 	if (output->base.zoom.active) {