Run RTL CSS urls through the style_loader_src filter

[nbachiyski]

Currently, if the rtl extra data for a stylesheet is set to true, its URL isn't generated by _css_href(), which applies the style_loader_src filter. Instead, the RTL URL is generated by the URL of the main stylesheet with .css replaced with -rtl.css.

Here is a patch, which properly gets the RTL URL using _css_href().

[ryan]

(In [10918]) Run RTL CSS urls through the style_loader_src filter. Props nbachiyski. fixes #9528