Speed up generateImage by re-using the outline/bitmap that has already been loaded.

Speed up generateImage by re-using the outline/bitmap that has already been loaded.

SkDrawText need to query both glyph metrics and image in most cases. But to query metrics and image separately will duplicate the work to load glyph outline which is time consuming especially for composite glyphs.

This patch record what is in the FreeType glyph slot and tries to re-use it. It get about 15% speed increasment with bench case fontscaler_lcd and fontscaler_aa (should be better for composite glyphs)

BUG=

[reed1, 2013-12-04 14:59:38]

Interesting patch. I will need some time to think about it.

Can you land the emboldenGlyph refactoring separately (in a different CL) ahead
of this change? That looks like a fine change in itself, and it would simplify
thinking about this glyph/context caching idea.

[bungeman-skia, 2013-12-04 15:46:59]

Note that I'm planning on landing bitmap font support soon, so many things will
change shortly. Hard to tell how that will affect this, I'll take a look at this
as soon as I have a handle on that.