LibCSS: new units and autogen script for the selection engine

I wanted to add some new length units to LibCSS, such as rem, vw, vh and others. The problem is that the css_unit value, which is stored in the style bits array, would need to gain a bit to accomodate the new values. Since many properties have length units, this change would require rewriting the files select/computed.h, propget.h and propset.h, because they need to be kept in sync.

My solution, in the git branch lcneves/select-autogen, is a Python script that autogenerates most of computed.h, propget.h and propset.h based on a simple configuration of properties and values. The bits array for each property group is automatically created by a "best fit" heuristics, which has produced optimal results for our data in all of my tests -- not guaranteed, though, since the bin packing problem is np-hard! This way, all three files are kept in sync.

Adding or modifying a property or value is just a matter of editing one line in the file select-config.py. If the property needs special functions in propget.h or propset.h, these can be provided in the file overrides.py (note that the index in the bits array and the bitwise mask and shift will still be autogenerated). Additional header and footer text for the autogenerated files goes in assets.py.

In the future, the script can be easily extended to autogenerate the files in select/properties/. Also, the script treats the groups "style" and "page" as special, but not "uncommon", so that adding more groups like "uncommon" can be done entirely in the config file -- but this would require autogenerating parts of computed.c and arena.c as well, to ensure that these groups are properly created and destroyed.

So, to sum it up: the branch libcss/lcneves/select-autogen features a generator script for a big chunk of the selection engine. Plus, 13 new length units.

Best,

Lucas