Fix readPixels for antialiased buffers.

When calling readPixels from javascript, we need to blit the
antialiased fbo into the render fbo before we can read the pixel.

Change-Id: I342342f396c42f3d5320105552838cd7c99d1c4b
Task-number: QTBUG-45373
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>