opengl: determine texture format from interop modules

Firstly, forward the format chosen by interop.c to the sampler, so that the sampler doesn't have to guess by performing the same checks.

Then, initialize the texture properties from the interop implementations. This effectively fixes #26735 (closed). As a consequence, opengl_interop_init() could be made internal to the interop_sw implementation.

Finally, improve the check to decide whether use GL_RED or GL_LUMINANCE based on the OpenGL version (in addition to the presence of the "GL_ARB_texture_rg" extension).

All these steps fix #26769 (closed) (in addition to #26735 (closed)).