Added GPUTextEngine (WIP)

[captain0xff]

This PR extends the new text rendering api to support the gpu api. It's still not ready for merging but the reason I am creating the PR is so that the API can be reviewed.

Things that I still need to do:-

• [x] The API needs to be documented (I will do this after the public API is finalized)
• [x] The math_3d.h header should be removed in favor of custom math
• [ ] Shaders should be added for all the backends along with a proper workflow to load and build them
• [ ] The textgputext example should be improved to show the full capabilities of the api

I will need some help with the shaders because I don't have much idea about the workflow needed for the Metal and Direct3D backends.