lib: move compositor to rust

[caiiiycuk]

Hi, here is my proposal for how we can transfer all graphics-related tasks to the render-ng library.

1. We need to change rv_init to accept a window context handle (provided by SDL; we can provide something from SDL_SysWMinfo). Like this:

struct rv_init_descriptor {
    uint32_t width;
    uint32_t height;
    const char* render_config;
    void* handle; // <----- native window handle
};

rv_context rv_init(rv_init_descriptor desc);

2. The function to render RGBA data in window coordinates (using the same coordinates as those used for rv_resize):

rv_render_rgba(const rv_rect& dest, uint32_t width, uint32_t height, uint8_t *rgba);

We also use 'texture coloring' (as in FragColor = texture(tex, uv) * color). While it's not mandatory, please add color as the last optional parameter if possible.

rv_render_rgba(const rv_rect& dest, uint32_t width, uint32_t height, uint8_t *rgba, uint32_t *color = nullptr);

The blend function is always the same: glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

3. lifecycle function:

enum Stage {
   LAYER_3D,
   LAYER_2D,
   PRESENT
}

rv_stage(Stage stage);

• LAYER_2D is called just before we start using rv_render_rgba. Here, you can set the blend function, viewport, etc.
• PRESENT marks the end of a frame, where we actually render to the frame.
• LAYER_3D is called immediately after PRESENT. Here you can reset the color buffer, etc.

4. Optiononal function that sets clear color (we want to change bg of the game window):

void rv_clear_color(uint32_t color)

P.S. Feel free to change the naming/declarations as you see fit.

[caiiiycuk]

It is very important to not break android version. Not sure if rust can create context on android device, so if possible let's keep fallback path of rv_init using glfunctor.