Add support for bitmap images

[mojca]

[mojca]

It is almost done now, except that:

• set output 'plots/myplot.tex' will generate externalfigure "plots/myplot_01.png" (you need to remove everything in front of the last slash and also test how that works on windows)
• externalfigure doesn't work in MKIV (only draw externalfigure works), but that needs to be fixed in context

[mojca]

Also, transparent images are still missing.