For this to work, you need to adjust your openbox configuration, assuming you are running a default Raspbian with LXDE / openbox. Edit .config/openbox/lxde-pi-rc.xml and add the following to the applications section:
<application class="scummvm" name="scummvm"> <fullscreen>yes</fullscreen> </application>
You can switch between fullscreen and windowed mode by pressing Alt+F11, in the default openbox configuration.
Next, you need to install scummvm and the OpenGL software rasterizer, since the touch screen frame buffer does not support the hardware OpenGL engine that the Pi provides:
$ sudo apt-get install scummvm mesa-utils libgl1-mesa-dri libgl1-mesa-swx11
This will also install dri drivers and some OpenGL utilities, such as glxgears to test performance.
Now you want to tell ScummVM to actually use the OpenGL backend driver, or else it will not scale to, but rather use the default 2x scaler, which is too large for the 480x320 display. Easiest way to do this is to edit ~/.scummvmrc (you have to have run ScummVM at least once for this) and edit the [scummvm] section to have these lines:
[scummvm] last_fullscreen_mode_width=480 last_fullscreen_mode_height=320 gfx_mode=opengl_nearest
If gfx_mode or the other lines already exist, edit them to your liking.