Merge: gamnit: fluid API, named 3D models, faster nitpick et other fixes

General improvements to the gamnit framework and related services:
* Make the APIs of `CustomTexture` and `VirtualGamepad` a bit more fluid.
* More precise computation of `TextSprite::height` and show the result in `font_showcase`.
* Optimize shadow mapping by caching the light MVP matrix.
* Speedup nitpick by silencing some warnings in `glesv2` and fix warnings created by the serialization framework.
* Fix bug on Kabylake GPU reported by @BlackMinou.
* Fix a few other bugs and possible crashes.

Pull-Request: #2551