app/debug: use the *image.RGBA drawing fast path.
app/debug: use the *image.RGBA drawing fast path.

On my 2013 Nexus 7, a simple

start := time.Now()
draw.Draw(etc)
log.Printf("draw.Draw took %v", time.Since(start))

around the draw.Draw call dropped from 12ms to 30µs.

Change-Id: I12aea4a4ade07eedb8ee86f7c8d4a4ed49f39f36
Reviewed-on: https://go-review.googlesource.com/11299
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

cainiaocome/mobile,goldcaddy77/mobile,qskycolor/mobile,0x90sled/mobile,ichu501/mobile,freeman0211/mobile,steeve/mobile,ppamorim/mobile,DanielWorld/mobile,lonphy23t/mobile,cainiaocome/mobile,xing/gomobile,b54898533/mobile,tempbottle/mobile,samanalysis/mobile,golang/mobile,0x90sled/mobile,goldcaddy77/mobile,0x90sled/mobile,tempbottle/mobile,tempbottle/mobile,b54898533/mobile,MaxCy330/mobile,ichu501/mobile,swisspol/mobile,golang/mobile,steeve/mobile,cainiaocome/mobile,steeve/mobile,samanalysis/mobile,samanalysis/mobile,0x90sled/mobile,zach-klippenstein/mobile,freeman0211/mobile,zach-klippenstein/mobile,xing/gomobile,golang/mobile,ichu501/mobile,b54898533/mobile,goldcaddy77/mobile,lonphy23t/mobile,qskycolor/mobile,MaxCy330/mobile,ppamorim/mobile,DanielWorld/mobile,cainiaocome/mobile,DanielWorld/mobile,lonphy23t/mobile,0x90sled/mobile,xing/gomobile,MaxCy330/mobile,steeve/mobile,xing/gomobile,ppamorim/mobile,zach-klippenstein/mobile,cainiaocome/mobile,swisspol/mobile,samanalysis/mobile,tempbottle/mobile,samanalysis/mobile,swisspol/mobile,lonphy23t/mobile,goldcaddy77/mobile,zach-klippenstein/mobile,freeman0211/mobile,swisspol/mobile,MaxCy330/mobile,ichu501/mobile,qskycolor/mobile,xing/gomobile,DanielWorld/mobile,swisspol/mobile,ppamorim/mobile,goldcaddy77/mobile,qskycolor/mobile,ppamorim/mobile,freeman0211/mobile,tempbottle/mobile,ichu501/mobile,golang/mobile,b54898533/mobile,b54898533/mobile,MaxCy330/mobile,lonphy23t/mobile