[Impeller] disable gl ext render to texture on vivante.

[jonahwilliams]

Found code in ANGLE that seems to indicate there are known crashes with vivnate drivers using gl_ext_render_to_texture. Landing to see if it fixes #167850 which we don't have a device to test for.

[flutter-dashboard]

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[jonahwilliams]

Plan is to land, ask developer in #167850 to test.

[rekire]

FYI: I can test the fix on Thursday on the device.