mesa: fix format selection for meta CopyTexSubImage()

When we do a glReadPixels into the temporary buffer, we don't want to
use GL_LUMINANCE, GL_LUMINANCE_ALPHA or GL_INTENSITY since they will
compute L=R+G+B which is not what we want.

This bug has existed all along but was only exposed by the elimination
of the driver hook for glCopyTexImage() in
5874890c26.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=39604
Tested-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/common/meta.c

@@ -2869,6 +2869,16 @@ copy_tex_sub_image(struct gl_context *ctx,
 
    /* Choose format/type for temporary image buffer */
    format = _mesa_get_format_base_format(texImage->TexFormat);
+   if (format == GL_LUMINANCE ||
+       format == GL_LUMINANCE_ALPHA ||
+       format == GL_INTENSITY) {
+      /* We don't want to use GL_LUMINANCE, GL_INTENSITY, etc. for the
+       * temp image buffer because glReadPixels will do L=R+G+B which is
+       * not what we want (should be L=R).
+       */
+      format = GL_RGBA;
+   }
+
    type = get_temp_image_type(ctx, format);
    bpp = _mesa_bytes_per_pixel(format, type);
    if (bpp <= 0) {