Optimisation: improve handling of custom wizard sprites

[lewster32]

Currently every wizard gets their own canvas-backed texture which blits a copy of the wizard template, alters the colours then appends the hat (if relevant). This could be improved in several ways:

• Append all generated wizard images to a single, shared texture atlas
• Completely ditch generated colour modification in favour of shader (a la the outline highlighting effect) with hats as 'child' sprites

The latter seems like it'd be the more efficient of the two, and allow for more flexibility.