Add Doom-style mouse/trackpad steering

[aversag]

What

Lets you steer the car with the mouse or trackpad, Doom-style. Press ` (backtick) or the Cmd/Win key in-game to grab the mouse — the cursor is hidden and locked in relative mode, and horizontal movement steers the car. Press the same key again to release it for the menus. Keyboard steering keeps working at the same time.

How

Implemented in the SDL harness (plus one small read-only helper), so no existing byte-matched game function is modified:

• Relative mouse motion accumulates into a "steering charge" that decays back to centre on every poll — so the wheel recentres when you stop moving, and a stationary mouse releases the steering even though SDL only sends motion events while the mouse is moving.
• That charge becomes a held steer-left/right key press by OR-ing the currently bound steer scancode into the keyboard state the game reads in GetKeyboardState. It follows the player's key mapping (arrows, WASD, …) instead of assuming a fixed binding. The underlying key_state is never modified, so real key presses are untouched and keyboard steering works alongside it.
• A new GetBoundScanCode() helper resolves the bound steer keys from gKey_mapping; it only reads game state.
• Mirrored across both the SDL2 and SDL3 drivers.

Config

Enabled by default.

• Disable: --no-mouse-steering, or MouseSteering = 0 in dethrace.ini.
• Sensitivity: --mouse-steering-sensitivity=1.5, or MouseSteeringSensitivity in the INI.

Notes

• The grab key accepts ` and Cmd/Win because backtick is a dead key on some layouts (e.g. macOS French), where the Cmd key is a reliable alternative.
• Built and tested in-game on macOS (arm64) with SDL2; also compiles cleanly against SDL3.