add PicoDDS to the autotools build
nullptr doesn't exist until C++11
stddef.h is needed to use size_t
old versions of GCC warn about headers that don't end with a newline
libpicodds.a needs to be linked after libgraphics.a
fix -Wswitch warning in TextureGL.cpp getMinSize
use ends_with to simplify & fix DDS file extension check

Note: this also makes the check case-sensitive, which we don't really want
but is consistent with the other file name/extension checks throughout the
code.
make file-local function static
print an error message to stderr if we try to load an invalid DDS
whitespace, typos, dead comment removal
fix typecast in TextureGL::Update when loading from DDS
LoadDDSFromFile: pass filename argument by const-ref
list PicoDDS/GTL in the third-party software section of AUTHORS.txt
switch to unsigned int for number-of-mipmap-levels

Chosen based on the guideline that 'int' should be the default integer type
if it has enough range (it does) and size isn't a concern (it isn't).
Merge pull request #30 from johnbartholomew/DDS

DDS