mesa/main: tighten rg/half-float interaction

The GL_HALF_FLOAT_OES-enum is about OES_texture_float, not e.g
ARB_texture_float. EXT_texture_rg does have an interaction that allows
this, but the other specs doesn't.

So let's tighten this. In reality, this shouldn't change any real
behavior, because we only support OES_texture_float in GLES contexts,
and in those we'd support EXT_texture_rg if we support RG textures in
the first place. But it makes the logic a bit clearer.

And just to be clear, the non-GLES version of the half-float enum does
not need the same check, because desktop GL supports all converions
here, and GLES 3 and later also requires RG-texture support in the first
place.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29528>

src/mesa/main/glformats.c

@@ -1882,7 +1882,7 @@ _mesa_error_check_format_and_type(const struct gl_context *ctx,
          return GL_NO_ERROR;
       case GL_RG:
       case GL_RED:
-         if (_mesa_has_rg_textures(ctx))
+         if (_mesa_has_EXT_texture_rg(ctx))
             return GL_NO_ERROR;
          FALLTHROUGH;
       default: