teximage: allow glCopyTex{Sub}Image[123]D into R/RG textures with OpenGL ES 2.0

This is explicitly allowed in the GL_EXT_texture_rg extension.

Cc: mesa-stable
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28313>

src/mesa/main/teximage.c

@@ -2476,6 +2476,15 @@ copytexture_error_check( struct gl_context *ctx, GLuint dimensions,
       case GL_RGB10_A2:
          break;
 
+      case GL_RED:
+      case GL_RG:
+         /* GL_EXT_texture_rg adds support for GL_RED and GL_RG as an internal
+          * format
+          */
+         if (_mesa_has_EXT_texture_rg(ctx))
+            break;
+
+      FALLTHROUGH;
       default:
          _mesa_error(ctx, GL_INVALID_ENUM,
                      "glCopyTexImage%dD(internalFormat=%s)", dimensions,