Add SDL_RenderGeometry functionality

[SynchroTuner]

Hello. SDL_RenderGeometry is added to rendering functionalities in SDL 2.0.18. It can render arbitrary triangle list directly within one call. Are there any plans to add such method to sdl2::render::Canvas?

The method signature may be pub fn geometry(&mut self, texture: &Texture<'_>, vertices: &[Vertex], indices: Option<&[u32]>), and the type Vertex is a wrapper of SDL_Vertex.

[kellerkindt]

Thankfully @moralrecordings has already a WIP implementation that works #1216

[Des-Nerger]

This functionality is so important for proper texturing in affine space (allowing for things like the smooth skinning in the 2D skeletal animation), why does the upstream wait for so long to accept the @moralrecordings's changes. I have no clue. I thought with v0.36.0 it'd be there.