misc release docs updates

lgt is finally dead dead
renamed DATADIR to DATA_DIR to be consistent everywhere

also makes the DATA_DIR cmakedefine work
CopyGemDataPath: indented preprocessor directives for readability
SDLVideo: install the shaders too and look for them

build dir still has precedence, so running without installing works
GLVideoDriver::createPrograms: display missing shader errors as fatal
Interface: don't discard the return value of CreateDisplay

it may fail and we should catch that
GLVideoDriver::~GLVideoDriver: don't crash if the shaders don't exist

now we finally exit gracefully