We want to store some metadata in our symbolic pngs, so make it
possible to get options when loading a png, along with the texture.
Update all callers.
Using libpng instead of the lowest-common-denominator
gdk-pixbuf loader. This will allow us to load >8bit data,
and apply gamma and color correction in the future.
For now, this still just provides RGBA8 data.
As a consequence, we are now linking against libpng.
