Add CMake build and cross-platform cleanup

BUILDING.md

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+# Building Motherload
+
+This repository now supports both the original Visual Studio solution files and a CMake-based build.
+
+## Supported build paths
+
+### 1. CMake on Windows
+Recommended if you want a modern, editor-friendly build setup.
+
+Requirements:
+- Visual Studio 2022 with Desktop development with C++
+- CMake 3.21+
+
+The repository already contains the required prebuilt Windows SDL packages in `Libraries/`.
+
+Configure and build:
+
+```bash
+cmake -S . -B build -G "Visual Studio 17 2022" -A x64
+cmake --build build --config Debug
+```
+
+### 2. CMake on Linux
+Requirements:
+- C++ compiler
+- CMake 3.21+
+- SDL2 development packages
+- OpenGL development package
+
+Ubuntu / Debian example:
+
+```bash
+sudo apt install build-essential cmake libsdl2-dev libsdl2-image-dev libsdl2-mixer-dev libsdl2-ttf-dev libgl1-mesa-dev
+cmake -S . -B build
+cmake --build build
+```
+
+### 3. Legacy Visual Studio solution
+The existing `.sln` and `.vcxproj` files are still in the repo and can still be opened directly in Visual Studio.
+
+## Build output
+
+The CMake build writes the executable to:
+- `build/bin/` for single-config generators
+- the active configuration output folder for Visual Studio generators
+
+Resource files from `Resources/` are copied automatically after a successful build.
+
+## Notes about dependencies
+
+### Windows
+CMake uses the bundled SDL package config files from:
+- `Libraries/SDLMain/.../cmake`
+- `Libraries/SDLImage/.../cmake`
+- `Libraries/SDLMixer/.../cmake`
+- `Libraries/SDLTtf/.../cmake`
+
+### Linux
+CMake falls back to `pkg-config` and expects these packages to be installed:
+- `sdl2`
+- `SDL2_image`
+- `SDL2_mixer`
+- `SDL2_ttf`
+
+## Troubleshooting
+
+### CMake fails at `find_package(OpenGL REQUIRED)`
+Install the OpenGL development package.
+
+Ubuntu / Debian:
+
+```bash
+sudo apt install libgl1-mesa-dev
+```
+
+### CMake cannot find SDL packages on Linux
+Install the SDL development packages listed above.
+
+### The game starts but cannot find assets
+Run the executable from the build output directory, not from the source directory root. The post-build step copies `Resources/` next to the executable.
+
+### Why are the old MSVC `#pragma comment(lib, ...)` lines still in the code?
+They are still there for the legacy Visual Studio project files, but the CMake build disables them explicitly so dependency management stays inside CMake.