add SDL_RenderTextureAffine

[expikr]

Closes #11279

This is the lowest-possible overhead API for planar texture copy, leaving the responsibility of calculating the points entirely to the users, allowing them to perform flips, stretches, shears, and rotations by calculating the basis vectors directly.

bool SDL_RenderTextureAffine(
    SDL_Renderer *renderer, 
    SDL_Texture *texture,
    const SDL_FRect *srcrect,
    const SDL_FPoint *origin,
    const SDL_FPoint *right,
    const SDL_FPoint *down
)

Type	Name	Description
SDL_Renderer *	renderer	the renderer which should copy parts of a texture.
SDL_Texture *	texture	the source texture.
const SDL_FRect *	srcrect	a pointer to the source rectangle, or NULL for the entire texture.
const SDL_FPoint *	origin	a pointer to a point indicating the location the top-left corner of srcrect should be mapped to, or NULL for the rendering target's origin.
const SDL_FPoint *	right	a pointer to a point indicating the location the top-right corner of srcrect should be mapped to, or NULL for the rendering target's top-right corner.
const SDL_FPoint *	down	a pointer to a point indicating the location the bottom-left corner of srcrect should be mapped to, or NULL for the rendering target's bottom-left corner.

[slouken]

Isn't this already achievable by using the RenderGeometry API?