Metal drawable size issue in readPixels

Running the modified `hello-pbr` of issue #4077 with a debug build of filament under Catalyst and doing a drag to trigger a `readPixels` leads to

_validateGetBytes:65: failed assertion `GetBytes Validation origin.x + size.width)(3024) must be <= width(2050).

which is produced by the `[readPixelsTexture ...` in `MetalDriver::readPixels` (and leads to a SIGABRT). The image looks fine on the screen.

On an actual iPad, this demo produces an image with a garbled strip on the right-hand side, as in #4077.