Required use
Closes #20 (closed).
Also pulling in some misc changes I'd made earlier. Mainly bugfixes, plus renaming DEPENDS to RDEPEND (to match the PMS and reflect that they are runtime dependencies), and removing the TEXTURE_SIZE variable from the Source, File and InstallDir classes, in favour of using the corresponding use flags.
Note that this breaks the repo and I'll push fixes to that after merging this.