There were several good arguments for this: it's how Windows works with
WM_NCHITTEST, SDL doesn't need to manage a list of rects, it allows more
control over the regions (how do you use rects to cleanly surround a circular
button?), the callback can be more optimized than a iterating a list of
rects, and you don't have to send an updated list of rects whenever the
window resizes or layout changes.
This fixes an issue where an empty cliprect is treated the same as a NULL
cliprect, causing the render backends to disable clipping.
Also adds a new API, SDL_RenderIsClipEnabled(render) that allows you to
differentiate between:
- SDL_RenderSetClipRect(render, NULL)
- SDL_Rect r = {0,0,0,0}; SDL_RenderSetClipRect(render, &r);
Fixes https://bugzilla.libsdl.org/show_bug.cgi?id=2504
