Add option to emulate mouse with analog sticks in libretro core

[desttinghim]

I've gotten WASM4 running on the RG351M, but there is no way for me to play games that require mouse input. We should add a libretro option to emulate the mouse with the analog stick and L1/R1.

[aduros]

I wonder if this could be something that can be built into RetroArch?

[desttinghim]

It probably could be. In fact, it sems kind of odd that it's not already a that it does.

[desttinghim]

Made an issue on the retroarch report https://github.com/libretro/RetroArch/issues/13555