opengl: interop_sw: fix >= 10bits rendering

Fixes a regression from 67bddf90

vlc_gl_interop_GetTexFormatSize() would always return an error for the
GL_R16UI / GL_RED combination. It's either GL_R16/GL_RED or
GL_R16UI/GL_RED_INTEGER.

But the sampler is using sampler2D and not usampler2D, so let's keep
GL_R16.