[10] RFR 8170024: PiscesRenderer.pr.emitAndClearAlphaRow cannot accept a pix_x_from parameter greater than clip.xmin

[Laurent Bourgès]

Please review this performance enhancement for OpenJFX 10 / Marlin:

JBS: https://bugs.openjdk.java.net/browse/JDK-8170024
webrev: http://cr.openjdk.java.net/~lbourges/marlinFX/marlinFX-8170024.0/

The performance impact is small but may be more important for large shapes
(left-side is 'empty) but anyway this patch only impacts the SW pipeline.

Changes:
- added a new method parameter pix_x_off to
PiscesRenderer.emitAndClearAlphaRow() giving the offset (in pixels) within
the alpha row + bound checks (for consistency)
-
*modified JPiscesRenderer to alter the initial alpha buffer pointer; the C
blit functions seems to support well the 'trick'*

*- use the new parameter in SWContext to make MarlinFX only process the
alphaRow in the range [pix_from; pix_to[ (as initially made for)*

Cheers,
Laurent