go.mobile/gl/glutil: use a triangle strip instead of two triangles.
go.mobile/gl/glutil: use a triangle strip instead of two triangles.

It's not a big deal in this case, but we'll probably want to do the same
for the sprite/gl package.

LGTM=crawshaw
R=crawshaw
CC=golang-codereviews
https://golang.org/cl/163990043

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