When possible, use UploadToScreen() rather than CompositePicture()
to upload glyphs onto the glyph cache pixmap. This avoids allocating
offscreen memory for each glyph making management of offscreen
areas much more efficient.
Add a function to composite multiple independent rectangles
from the same source to the same destination in a single
operation: this is useful for building a glyph mask.
Add back exaGlyphs(); the new version copies the glyph images
onto a single large glyph pixmap and draws from their to the
destination surface. This reduces the management of small
offscreen areas and will allow us to avoid texture unit setup
between each glyph.
