When we are loading symbolic pngs or svgs, we know
that we need to the png or svg loader, so there is
no need to go through (surprisingly expensive) mime
sniffing to find the right loader.
When we are loading themed icons, we know if
we deal with an svg or png file, so it is
entirely unnecessarily to have gdk-pixbuf
use g_content_type guess to rediscover that
information.
Change the pixbuf utils apis we have to allow
passing format information down to where we
can use it when creating the pixbuf loader.
With the shader approach to symbolic recoloring, we must
not recolor the svgs anymore as we're loading them. Instead,
load them the same way that gtk-encode-symbolic-svg does.
This fixes the rendering of large symbolic icons e.g. the
'no search results found' page in the file chooser.
When creating icon info objects for unthemed files, we don't
really have a nominal size, so we pass 0 to mean 'load at
original size'. However, this is not what was happening.
To make this possible, add variants of some pixbuf loading
functions that take a scale factor instead of a desired size,
and use those when we don't have a nominal size.
