pixman-renderer: Fix read_pixels for screen recorder

The old code had an off-by-one error on the y coordinate
where it says height - (cur_y - y). And it does the vflipping of
the *destination* buffer, whereas what is really needed is to
vflip the whole source buffer. This only affects when you read
out part of the image, such as when using the screen recoder.

Also, instead of doing the flipping manually we just let pixman
handle it.

src/pixman-renderer.c

@@ -72,8 +72,8 @@ pixman_renderer_read_pixels(struct weston_output *output,
 			       uint32_t width, uint32_t height)
 {
 	struct pixman_output_state *po = get_output_state(output);
+	pixman_transform_t transform;
 	pixman_image_t *out_buf;
-	uint32_t cur_y;
 
 	if (!po->hw_buffer) {
 		errno = ENODEV;
@@ -86,18 +86,25 @@ pixman_renderer_read_pixels(struct weston_output *output,
 		pixels,
 		(PIXMAN_FORMAT_BPP(format) / 8) * width);
 
-	/* Caller expects vflipped image */
-	for (cur_y = y; cur_y < y + height; cur_y++) {
-		pixman_image_composite32(PIXMAN_OP_SRC,
-			po->hw_buffer, /* src */
-			NULL /* mask */,
-			out_buf, /* dest */
-			x, cur_y, /* src_x, src_y */
-			0, 0, /* mask_x, mask_y */
-			0, height - (cur_y - y), /* dest_x, dest_y */
-			width, /* width */
-			1 /* height */);
-	}
+	/* Caller expects vflipped source image */
+	pixman_transform_init_translate(&transform,
+					pixman_int_to_fixed (x),
+					pixman_int_to_fixed (y - pixman_image_get_height (po->hw_buffer)));
+	pixman_transform_scale(&transform, NULL,
+			       pixman_fixed_1,
+			       pixman_fixed_minus_1);
+	pixman_image_set_transform(po->hw_buffer, &transform);
+
+	pixman_image_composite32(PIXMAN_OP_SRC,
+				 po->hw_buffer, /* src */
+				 NULL /* mask */,
+				 out_buf, /* dest */
+				 0, 0, /* src_x, src_y */
+				 0, 0, /* mask_x, mask_y */
+				 0, 0, /* dest_x, dest_y */
+				 pixman_image_get_width (po->hw_buffer), /* width */
+				 pixman_image_get_height (po->hw_buffer) /* height */);
+	pixman_image_set_transform(po->hw_buffer, NULL);
 
 	pixman_image_unref(out_buf);