Optimize glyph cache texture uploads

Only upload the changed area of the glyph cache, not the entire
bitmap. Note that we can't do the full-on optimization here of copying a sub-rect
of the bitmap because of GL ES 2 limitations, but we can at least copy the
horizontal stripe containing the dirty rect, which can still be a big
savings over uploading the entire bitmap.

Issue #7158326 Bad framerates on MR1 (Mako, Manta, Prime)

Change-Id: Iab38d53202650f757ead4658cf4287bdad2b3cb9