mesa: fix texstore with GL_COLOR_INDEX data

This got broken by 3dbf5bf657.
GL_COLOR_INDEX data is still supported (in legacy contexts), but the new
texstore_swizzle path cannot handle it (and didn't detect this).
Unfortunately there's no piglit test trying to specify textures with a
GL_COLOR_INDEX source format, and I don't really understand how all the color
map stuff which is used by this works, but this caused conform failures
(with a reported mesa implementation error when trying to figure out the color
mapping).

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/mesa/main/texstore.c

@@ -1495,6 +1495,9 @@ texstore_swizzle(TEXSTORE_PARAMS)
    if (!is_array)
       return GL_FALSE;
 
+   if (srcFormat == GL_COLOR_INDEX)
+      return GL_FALSE;
+
    switch (srcType) {
    case GL_FLOAT:
    case GL_UNSIGNED_BYTE: