MatrixForeground - make variables static

Variables not used elsewhere, this protects them from accident.
However, they probably should be instance variables in the
smartmatrix class (or some related class).
(Bad things would happen if you made more than one smartmatrix).
make globals static - MatrixGraphics.cpp MatrixFont.cpp SmartMatrix.cpp
mark const - gamma tables and get functions
SmartMatrix - fill in prototype
Add const to arguments (color, pointers)

Safer, may be more efficient
Pass colors by reference

Should not need to make as many copies
BitmapTest - error checking (be sure to not read beyond end of bitmaps)
readpixel - make compatible with pre C++11 syntax
COLOR_DEPTH_RGB > 24 - use defines to condense duplicate code

Easier to see what different in the two versions
Less chance of changing code one place and not the other.
Would need 16 bit per chanel for color depths between 24 and 36
Merge pull request #15 from ecdr/ConstStatic

misc - const, static, pass colors by reference