Add support for cursor capture and relative mouse motion

[cyberarm]

• Mouse capture via SDL_SetWindowGrab
• Relative mouse motion via SDL_SetRelativeMouseMode

Useful for windowed mouse look games like jcaster, i-mic-fps, etc.

[cyberarm]

@jlnr What are your thoughts on this PR?

• The hit test callback will enable borderless windows to be draggable and resizable
• The capture_cursor callback will enable applications that rely on the mouse being confined to the window area (e.g. FPS camera controller without jumping from lost input because mouse left window)

[jlnr]

@cyberarm Mixed:

• I love the idea of capturing the cursor.
• After using Macs for 15 years, having a "maximize" function seems very Windows-like, and I'd have to check how it even works on macOS. (Usually apps just use fullscreen mode.) I'm also not sure when an app that I used ever minimized or restored itself. What are the use cases for it (sorry if I've asked this before)?
• Similarly skeptical about the hit-testing use cases, and having another enum in Gosu is not something I am looking forward to :/

I guess the dividing line is that I'd merge anything that helps building great 2D games for desktop and mobile, but I need some convincing with almost-finished projects for anything that goes into the UI toolkit area.

[cyberarm]

• After using Macs for 15 years, having a "maximize" function seems very Windows-like, and I'd have to check how it even works on macOS. (Usually apps just use fullscreen mode.) I'm also not sure when an app that I used ever minimized or restored itself. What are the use cases for it (sorry if I've asked this before)?
• Similarly skeptical about the hit-testing use cases, and having another enum in Gosu is not something I am looking forward to :/

Having minimize, maximize and restore functions would be useful for windowed borderless applications and games that want a fully custom appearance, e.g. the TimeCrafters Configuration Tool that I wrote for the robotics team would benefit from both the hit-testing and min/maximize functions.

[jlnr]

Aah, I see! Makes sense that borderless windows would need a way to recreate the window decorations. I have to say I prefer how some portable apps (Spotify comes to mind) have native macOS window controls in a non-native window. I've never looked into how it works, but that seems much nicer than manually recreating controls that don't respect the rest of the OS (left vs right window edge, some distros don't have a minimize button, etc.)

(It doesn't seem to be trivial because Spotify doesn't do the same thing on Linux :/ It'd probably have to be in SDL2 proper for me to consider it)

Anyway, that's pretty deep in UI toolkit land for me. I'd prefer to only merge the cursor capture for now.

[cyberarm]

@jlnr I've removed the window controls and hit testing from this PR.
I think I added everything needed to merge except for a git diff patch for capture_cursor? ➡️ protected_capture_cursor? in gosu_SWIG_RENAME_PATCH.patch

[jlnr]

1. Can you please rebase on master?
2. Hmm, isn't this PR missing something to work for something like a 3D shooter? Because if you were to implement mouse camera movements, you wouldn't just want the cursor to be confined to the window, because then once you hit any edge, you cannot move any further. Probably not an issue in other frameworks that have a mouseMove(dx, dy) event, but Gosu doesn't. :/ Or is the intention that users of capture_mouse still need to move the mouse back to the window center every tick?

[cyberarm]

Or is the intention that users of capture_mouse still need to move the mouse back to the window center every tick?

This was the original intention, however we could use SDL_SetRelativeMouseMode and add Window#relative_mouse_x/y to accomplish the same with perhaps more consistent/smoother mouse movement since the mouse doesn't need to be re-centered each update.

SDL_SetRelativeMouseMode does hide the cursor and prevent it from "moving" according to the wiki, so it might be desirable to keep capture_cursor and add another callback, perhaps Window#relative_mouse_motion to enable/disable relative mouse motion.