Fix for rtt ios

Hi bros,

I took some time to send a PR to youto enable  nme/lime/openfl users to have properly fuctionning render to texture.

Basically I snatch the "root framebuffer" in ios. This is mandatory as ios does not have an opengl compliant policy. (app backbuffers are not the framebuffer 0)

note : My commit are ugly because merging files with 3-space as tabs ( erk)+ git merge = deah...

Thanks.