Fix backcompat with wx 2.8. This involved writing
wxPositiveDoubleValidator and rewiring the DrawingPanel inheritance tree
and event handling mechanisms (because 2.8 does not have ->Bind, only
->Connect which is less flexible.) As a result all the event handling
has been gathered into GameArea and the affected code is somewhat
cleaner. 2.8 support is untested on Mac because it requires 32 bit libs
and Carbon.

Add support for cross-compiling for windows using the Fedora MinGW
packages in ./installdeps.

Check for OpenGL support in the wx library being linked, this was
necessary because the Fedora MinGW wx library does not have OpenGL
support.

Remove vbamDebug() in favor of wxLogDebug(), and add an override for it
so that it works in non-debug builds of wx as well as on Windows. Turn
off buffering on stdout and stderr on startup so that debug logging
works in msys2/cygwin mintty as well.

On Windows, build a console binary for debug builds.

Update README.md to reflect Fedora MinGW support and debug logging
support.

Add -Wextra to cflags for debug builds.