Pytvpaint

Features :

- ADD background mode option in render functions
- UPDATE background mode and color functions in API

Fixes :

- FIX render context setting save mode instead of alpha mode

Thanks to Kim Strandli for reporting this 👍

Features:

* ADD new functions to Layer and LayerInstance
* ADD new functions and properties to make range handling easier


Fixes:

* FIX misc issues with rendering
* FIX and UPDATE documentation