Updated distance field glyph caches to use QDistanceField.

Some OpenGL drivers assume alpha-only images are always tightly packed,
regardless of what is set for GL_UNPACK_ALIGNMENT. We now use
QDistanceField to store glyphs, which aligns scanlines on a 1-byte
boundary, instead of QImage which uses a 4-byte boundary. A previous
workaround uploaded scanlines one at a time, but this is also broken
with some other drivers...

Task-number: QTBUG-30908
Task-number: QTBUG-32861
Change-Id: I46527fb48de1e00116f776427c45baa752c6176d
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>