glupload: Do not expose external only format/modifiers pairs if GL_OES_EGL_image_external is not supported
At the moment, we expose all format/modifiers pairs. Though, on AMD I notice that all these pairs are external only, which can only be used if GL_OES_EGL_image_external (aka with GST_GL_API=gles2). As a side effect, if upstream starts producing these, we won't actually be able to import it and negotiation will fail.
Instead, we should ignore external_only modifiers if when we don't have GL_OES_EGL_image_external support. A special case should be made for linear modifier (0) so the emulate DMAbuf importer (using alternative texture format) can be used.
Long term, we could possibly improve OpenGL support by checking if the modifier can be used with the emulated (aka R8/16 and RG88/1616 formats). To be validated with the GFX folks, as this might be a bit ambiguous.