Workaround for distance field glyph upload problem.

It seems uploading an image (into a texture) which has
padding at the end of each scanLine and alpha-only pixel format
is broken with some OpenGL ES drivers.
The workaround is to upload one line at a time.

Task-number: QTBUG-30908
Change-Id: Ic680654951b6aec294c1a173708c1fb75e57ff8f
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>