optimize blend32_16_row and unroll SkRGB16_Blitter::blitRect
optimize blend32_16_row and unroll SkRGB16_Blitter::blitRect

Reference benchmark using 0xbench 2D on Nexus S (1 GHz)

[before]
Draw Rect:      28.52 fps

[after]
Draw Rect:      37.89 fps

Change-Id: Ibfb8e38ddfe6df9887f15c48027cf0685f317152

freerunner/platform_external_skia,freerunner/platform_external_skia,freerunner/platform_external_skia,freerunner/platform_external_skia