Fix compiler warnings

Fix the following warnings, seen when compiling with GCC 5.4:

    CXX OLEDDisplay.o
    ./OLEDDisplay.cpp: In member function 'void OLEDDisplay::drawStringInternal(int16_t, int16_t, char*, uint16_t, uint16_t)':
    ./OLEDDisplay.cpp:381:10: error: enumeration value 'TEXT_ALIGN_LEFT' not handled in switch [-Werror=switch]
       switch (textAlignment) {
	      ^
    ./OLEDDisplay.cpp: In member function 'void OLEDDisplay::drawInternal(int16_t, int16_t, int16_t, int16_t, const char*, uint16_t, uint16_t)':
    ./OLEDDisplay.cpp:722:13: warning: unused variable 'yScreenPos' [-Wunused-variable]
	 int16_t yScreenPos = yMove + yOffset;
		 ^
    ./OLEDDisplay.cpp: In member function 'void OLEDDisplay::drawXbm(int16_t, int16_t, int16_t, int16_t, const char*)':
    ./OLEDDisplay.cpp:361:19: error: 'data' may be used uninitialized in this function [-Werror=maybe-uninitialized]
	     data >>= 1; // Move a bit
		       ^
    cc1plus: some warnings being treated as errors