texgen: second iteration

[Helco]

So long story short, this makes texture generation much more useful and useable.

• [x] Fixes some bugs in texgen
• [x] Makes texgen much more dynamic
• [x] Add texgen color parameter type
• [x] Add brickwall texture generator
• [x] Add pcmockup texture window (to view, import or generate textures)

As per usual with this size of PR there are some refactorings (e.g. UploadedTexture) which I will extract into separate PR's and wait for them to be merged.