render/pixman: Fix non-uniform scale with rotation

e08d52bb introduced a bug when rotating by 90 or 270 degrees and doing
non-uniform scaling (different scale factor on X and Y axes).  The scale
factor was calculated as the ratio between the src box and the rotated
dst box.  But scaling is applied after rotation, so the scale factor
should instead be the ratio between rotated src box and dst box.

render/pixman/pass.c

@@ -70,14 +70,14 @@ static void render_pass_add_texture(struct wlr_render_pass *wlr_pass,
 		});
 	}
 
-	// Rotate the final destination size into source coordinates
+	// Rotate the source size into destination coordinates
-	struct wlr_box orig_box;
+	struct wlr_box src_box_transformed;
-	wlr_box_transform(&orig_box, &dst_box, options->transform,
+	wlr_box_transform(&src_box_transformed, &src_box, options->transform,
 		buffer->buffer->width, buffer->buffer->height);
 
 	if (options->transform != WL_OUTPUT_TRANSFORM_NORMAL ||
-			orig_box.width != src_box.width ||
+			src_box_transformed.width != dst_box.width ||
-			orig_box.height != src_box.height) {
+			src_box_transformed.height != dst_box.height) {
 		// Cosinus/sinus values are extact integers for enum wl_output_transform entries
 		int tr_cos = 1, tr_sin = 0, tr_x = 0, tr_y = 0;
 		switch (options->transform) {
@@ -123,10 +123,10 @@ static void render_pass_add_texture(struct wlr_render_pass *wlr_pass,
 
 		// Apply scaling to get to the dst_box size.  Because the scaling is applied last
 		// it depends on the whether the rotation swapped width and height, which is why
-		// we use orig_box instead of dst_box.
+		// we use src_box_transformed instead of src_box.
 		pixman_transform_scale(&transform, NULL,
-			pixman_double_to_fixed(src_box.width / (double)orig_box.width),
+			pixman_double_to_fixed(src_box_transformed.width / (double)dst_box.width),
-			pixman_double_to_fixed(src_box.height / (double)orig_box.height));
+			pixman_double_to_fixed(src_box_transformed.height / (double)dst_box.height));
 
 		// pixman rotates about the origin which again leaves everything outside of the
 		// viewport.  Translate the result so that its new top-left corner is back at the