webengine: Fix slow video with webengine on imx6 devices

Disable texture_rg on imx6 based devices. Yocto image 1.8 has driver,
which reports opengl es 3.0 support, however texture_rg does not work
properly.

Change-Id: Iaeadbfb16bacd3d78d9c660fa02e761e1781b4f3
Reviewed-by: Teemu Holappa <teemu.holappa@theqtcompany.com>