Rendering to different default gl framebuffers - sokol_gfx.h

[danielchasehooper]

In the OpenGL build of our app, each document window has its own CAOpenGLLayer. I've looked into using sg_contexts in order make sg_begin_default_pass render to the correct CAOpenGLLayer framebuffer[^1]. Unfortunately sg_contexts affect resource pooling, which I do not want. All documents share the same glyph buffer and other resources for common visual elements, and I don't want those resources to be destroyed when one document is closed[^2]. I do not want changing the default pass's framebuffer to affect anything to do with resource lifetimes.

I propose adding:

typedef struct sg_gl_context_desc {
    const GLuint (*default_framebuffer_cb)(void);
    const GLuint (*default_framebuffer_userdata_cb)(void*);
    void* user_data;
} sg_gl_context_desc;

typedef struct sg_context_desc {
    ...
    sg_gl_context_desc gl;
} sg_context_desc;

That way you can return the desired framebuffer via that callback, and would match the apis of the other graphics backends[^3].

Related to #885

[^1]: Also I haven't seen any documentation guaranteeing that the framebuffer that is bound in drawInCGLContext:pixelFormat:forLayerTime:displayTime:/CGLSetCurrentContext is always the same. So even if resources weren't a concern with sg_context, it seems possible that the framebuffer bound during sg_setup_context might later not be the same framebuffer when sg_begin_default_pass is called

[^2]: I have all CAOpenGLLayers use the same CGLContextObj for this reason.

[^3]:I don't love callbacks since they're harder to read than straight-line code. I think there could be a nicer API along the lines of sg_begin_pass(sg_default_pass(&(sg_default_pass_desc){ .gl.framebuffer = fbo, .width = width, .height = height }), &pass_action) This would allow writing functions that accept a pass as a parameter and that pass could target either an offscreen buffer or the framebuffer; all without affecting the receiving function's implementation. This would also improve #885.