pixman-renderer: Fix read_pixels for screen recorder

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.