Current master is failing due to libglew-dev being required
even if opengl is disabled. As a temporary measure, install
package to satisfy build (and raise the issue upstream later).
Add experimental DOSBox port with SDL2 & FluidSynth support
Thanks to: https://github.com/duganchen/dosbox
Preliminary findings:
* Performance is roughly equal to vanilla build (~11fps with Quake +timedemo demo1 on both)
* Graphical tearing no longer appears to be present (or is greatly reduced)
* Better scaling of the display to the entire screen
* The blue colour bug when opening the mapper via Ctrl+F1 is no longer present.
Disadvantages:
* CD-ROM emulation is not available due to lack of support in SDL2 (does not affect IMGMOUNT,
so won't be a problem on Raspberry Pi)
* Mapper and windowed mode does not scale to full screen (but the port is configured to
use fullscreen by default to mitigate the problem).
