Optimised hwui rounded corners shader

This is a ~2x performance increase for the rounded corners shader which
was previously shader limited even on many high-end GPUs. It simplifies
the calculations by pre-dividing by the radius, which also allows to
maintain mediump/FP16 precision (this improves performance and might
improve image quality on GPUs that don't support highp e.g. Mali-400).
It also uses a conditional in the shader to save much of the work on
the vast majority of pixels which are not close to the corners.

Test: manual - visual inspection on fugu (nexus player)

Change-Id: I38f491cfb006e2991ef0b9d6b3d67d2c4d328d44