#8 misc patches

[Roine Gustafsson]

This is a big 37-patches pack I've had sitting on my HD for a long time. I've brushed it up and they should apply cleanly to the current SVN.
Most are smaller fixes but some are new functionality.

Notable stuff:
- improved error message context
- background imager shader support
- support for older PRman 11 'trace' and 'transmission' syntax
- texmake image importers from QuickTime for jpg/png/tga/... (Mac OS X only)
- some speedups from removing double-to-float conversions

[Roine Gustafsson]

zip with patches