Less aggressive glyphs precaching

The renderer used to pre-cache glyphs at record time. This then changed
to pre-caching at the beginning of every frame. This unfortunately entails
a lot of duplicate work on every frame, which amounts to 0.5 to 1ms in
some stock applications.

This change is somewhere in the middle: pre-caching happens the first
time a DrawTextOp is deferred or every time the screen-space transform
is different from the last pre-caching operation.

Change-Id: Id6d9e2599d90a5b75010b0f0a28746befbf3c205