SDL WASD + mouse spectator piloting

[marceline-cramer]

Dependent on #19

Using SDL's keyboard and mouse input, update a viewports view matrix each frame to make it fly around in 3D.

Prototype engine reference implementation:

• https://github.com/mondradiko/mondradiko/blob/main/core/displays/SdlDisplay.cc#L336
• https://github.com/mondradiko/mondradiko/blob/main/core/avatars/SpectatorAvatar.cc#L38