Added resampled height field to ground's collision shapes.

The resampling truncates details so that 1/16^2th of meshes of
original height field are reserved, but still it takes too much
time to cache the interpolated height, especially in debug
build.
SurfaceCS is made to accept file names for TIN.

The file name can be given in .ssd files.
surface: Sped up loading TIN and its resampled DEM.

TIN-resampled DEM is cached into cache subdirectory at the first
invocation of the program.
Repeated startup of the program loads the cached data to shortcut
the calculation needed to resample TIN.

This workaround is not really a right way.  We should enhance
the algorithm to quickly detect a triangle that contains a given
point.  But it's known to work and really effective (except for
the first run, of course).
surface: Made TIN resampling frequency configurable in a .ssd.
cpplib: Added Mat2 class template.
surface: Fixed algorithm to calculate TIN's interpolated height.

We had to obtain inverse matrix to find coordinates local to each
triangle.