common paint: alpha masking optimization

common paint: alpha masking optimization

applying fast track approach to the alpha mask when its condition is matched.
(simple rectangle alpha masking which isn't half-translucent)

From time to time, designers brutally use the alpha masking to clip simple region,
thorvg can help the situation by avoiding masking usage internally.

This can reduce the 1-step render pass composition.

@Issue: https://github.com/Samsung/thorvg/issues/344