vo_opengl: GLES does not support GL_BGRA

Apparently GLES 2 and 3 do not support this. (The implementations I
tested with were derived from desktop OpenGL and were not overly strict
with this.)

This is no problem; just use GL_RGBA and mangle the channels in the
shader.

Also disable direct support for image formats like IMGFMT_RGB555 with
GLES; at least some of them are not supported in this form, and the
formats aren't important anyway.