Merge: gamnit: new services and a lot of bug fixes and performance improvements

Merge: gamnit: new services and a lot of bug fixes and performance improvements

This PR groups small fixes to gamnit and related packages. It is a general cleanup in preparation for optimizations to the gamnit depth 3D API.

Intro a few new services:
* `ParticleSystem::clear` to remove all live particles.
* `CustomTexture` can be modified and reloaded in GPU memory after the first call to `load`.
* Don't apply dynamic resolution to UI sprites as they are usually lightweight and any change in resolution is easily noticable.

Optimizations:
* Matrix creation and manipulation, in both gamnit and the matrix package.
* Free pixel data from both Nit and Java memory after loading them from the assets folder into the GPU memory.
* Avoids using mallocs in `realtime` services.

Fixes:
* Fix the left and right anchors of the `UICamera`.
* Fix a constant used to ask for antialiasing in the `egl` package.
* Avoid long attribute names in Blinn-Phong shader in case their name is truncated or the attribute is optimized out.
* Fix pointer to the parent GPU texture name in subtextures.
* Improve a few API doc.

Pull-Request: #2586
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>
Reviewed-by: Jean Privat <jean@pryen.org>