d3dretrace: Make ~CDXGI* virtual.

Silences `deleting object of polymorphic class type ‘...’ which has
non-virtual destructor might cause undefined behaviour` GCC warning.
retrace: Pass additional image information via struct on JSONWriter::writeImage.

Stepping stone to pass more detailed image information to the GUI.
md5: Move `extern "C"` into the md5.h header.
md5: Set -fno-strict-aliasing flag where available.

Fixes #309.

Should probably use for all source code.
cli: Cleanup pager code.

And silence a gcc warning.
cli: Silence unused copyWrapper function warning.
retrace: Silence unused functions warnings.
image: Address uninitialised variable warning.