Fix buffer blit matrices

There was a missing wlr_matrix_scale call, so we ended up with black
frames.

Closes: https://github.com/swaywm/wlroots/issues/2780

backend/drm/renderer.c

@@ -157,6 +157,7 @@ static struct wlr_buffer *drm_surface_blit(struct wlr_drm_surface *surf,
 
 	float mat[9];
 	wlr_matrix_identity(mat);
+	wlr_matrix_scale(mat, surf->width, surf->height);
 
 	wlr_renderer_begin(renderer, surf->width, surf->height);
 	wlr_renderer_clear(renderer, (float[]){ 0.0, 0.0, 0.0, 0.0 });

types/wlr_screencopy_v1.c

@@ -287,6 +287,7 @@ static bool blit_dmabuf(struct wlr_renderer *renderer,
 
 	float mat[9];
 	wlr_matrix_identity(mat);
+	wlr_matrix_scale(mat, dst_buffer->width, dst_buffer->height);
 
 	wlr_renderer_begin(renderer, dst_buffer->width, dst_buffer->height);
 	wlr_renderer_clear(renderer, (float[]){ 0.0, 0.0, 0.0, 0.0 });