Snap native glyphs to pixel grid in vertex shader.

The implementation relied on the full matrix, but did not
set the RequiresFullMatrix flag. Setting the flag would
have serious negative performance impact as it prevents
batching, so we solve it in the vertex shader instead.

Task-number: QTBUG-38702
Change-Id: I0c245ea9e18b0b29dd9e3073a2648a7f4e061685
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>