vector: add a fast path for RGBA dst images.

name                      old time/op  new time/op  delta
GlyphRGBA16Over-8         25.1µs ± 2%   6.0µs ± 0%  -76.09%  (p=0.000 n=10+10)
GlyphRGBA16Src-8          17.3µs ± 2%   5.4µs ± 0%  -68.68%  (p=0.000 n=10+10)
GlyphRGBA32Over-8         93.2µs ± 2%  14.5µs ± 0%  -84.41%  (p=0.000 n=10+10)
GlyphRGBA32Src-8          59.3µs ± 2%  12.4µs ± 0%  -79.04%    (p=0.000 n=9+9)
GlyphRGBA64Over-8          350µs ± 2%    45µs ± 0%  -87.21%    (p=0.000 n=9+9)
GlyphRGBA64Src-8           223µs ± 2%    37µs ± 0%  -83.58%    (p=0.000 n=9+9)
GlyphRGBA128Over-8        1.37ms ± 2%  0.16ms ± 1%  -88.19%   (p=0.000 n=9+10)
GlyphRGBA128Src-8          868µs ± 2%   128µs ± 0%  -85.21%   (p=0.000 n=10+9)
GlyphRGBA256Over-8        5.50ms ± 3%  0.59ms ± 0%  -89.31%  (p=0.000 n=10+10)
GlyphRGBA256Src-8         3.47ms ± 2%  0.45ms ± 0%  -86.91%   (p=0.000 n=9+10)

Change-Id: I3df60e6b7147872367715361c9d1ed52951b22e0
Reviewed-on: https://go-review.googlesource.com/29699
Reviewed-by: David Crawshaw <crawshaw@golang.org>
1 file changed