xmake based buildsystem

[PerikiyoXD]

Not an issue, more of an RFC.

Completed an xmake port for librw. Single xmake.lua at root, plus an xrepo package definition for consumers.

What's included:

• All backends: null, gl3 (glfw/sdl2/sdl3), d3d9
• lodepng configurable: builtin (in-tree src/lodepng/lodepng.cpp) or fetched via xrepo
• All tools and skeleton built correctly across all combos
• Output separated by triplet + backend/gfxlib combo (e.g. build/bin/windows-x64-release-gl3-sdl2/)
• xrepo consumer package at packages/l/librw/xmake.lua

Tested on Windows x64 with MSVC, all 10 combos (5 backends x 2 modes) build successfully.

Happy to open a PR if there is interest.