Merge: gamnit: intro static particles, Sprite tint & invert_x, and fix a few bugs

Intro a few new features:
* `Sprite::tint` to tint the color of a sprite.
* `Sprite::invert_x` to draw a sprite with the X axis inverted.
* `App::static_program` a particle program to display simple static and long lived particles. Can be useful to display clouds.

Fix a few bugs:
* `svg_to_png_and_nit` generated invalid code for array of textures when the target was gamnit.
* `App::clock` was instantiated with `app`, this caused the first `dt` of `App::update` to be much larger than expected.
* Divide `frame_core_flat` in 2 to allow the depth framework to draw particles after the world sprites but before the UI sprites.

Pull-Request: #1934
Reviewed-by: Jean Privat <jean@pryen.org>