Another optimization of glyph cache uploads

Previously, cache textures were updated whenever mCurrentCacheTexuture was changed.
Since updating cache textures needs glTexSubImage2D call, frequent changing
of mCurrentCacheTexture (which can easily happen when an app uses lots of unique glyphs
even with precaching) caused many glTexSubImage2D calls and bad framerates.
This patch optimized isssueDrawCommand function. Consequently, changing mCurrentCacheTexture doesn't
cause glTexSubImage2D call any more and it will improve font rendering performance.

Change-Id: Id19d959fa0e69eeb2a39f83a57e311d7394586b2
Signed-off-by: Sangkyu Lee <geteuid@gmail.com>