fix(browser): warm only the font sources the renderer will consider

prepare_screenshot_resources collected its work with css_resource_urls,
which returns every url() in the stylesheet text. A @font-face src list
is a priority order, not a set, so the common IE8 form produced six
downloads for one font.

The renderer already resolves that list correctly. font_face_declaration
in obscura-render ends in .last(), so a rule with several src descriptors
uses the final one, and font_source_may_be_supported drops .eot and .svg
after stripping the query and fragment. The warmup went through neither,
so it fetched bytes the renderer had already ruled out.

css_font_face_rule applies both rules and reports the block as consumed,
the same shape as the existing @import skip in the same scanner. On the
reported fixture that is six fetches down to three.

The url recording moves into push_css_url so the generic scan and the
font path cannot disagree about quoting, fragments, data: or var().

An unterminated block returns None and is left to the generic scanner,
matching css_import_rule_len: warming too much is better than dropping
the rest of the stylesheet.

Fixes #667.