From dhewm3: Add absolute mouse mode and refactor mouse grabbing code

The only change really is adding SE_MOUSE_ABS to sysEventType_t.
In theory this breaks the ABI, but in practice I don't think mods use
sysEventType_t, and certainly not the ones above SE_MOUSE
(SE_JOYSTICK_AXIS doesn't make sense as Doom3 never supported Joysticks,
 SE_CONSOLE is for input from TTY or whatever, and doesn't make sense
 for mods to use in any way)

The change in Stub_SDL_endian.h is just me being paranoid.
(side-note: it *might* make sense to replace all that inline-asm in
 Stub_SDL_endian.h with compiler intrinsics like GCC/clangs
 __builtin_bswap* and MSVCs _byteswap_* - if they are supported in
 all relevant compiler versions. GCC supports 32 and 64 bit swaps since
 4.3, 16bit since 4.8 and 128bit since 11.x; clang supports 32 and
 64 bit swaps at least since 3.0 and 16bit swaps since 3.2;
 VS introduced _byteswap_ushort, .._ulong and .._uint64 in VS2003)

sys/Stub_SDL_endian.h

@@ -34,6 +34,10 @@
 #define SDL_LIL_ENDIAN  1234
 #define SDL_BIG_ENDIAN  4321
 
+#ifndef BUILD_IS_BIG_ENDIAN
+  #error BUILD_IS_BIG_ENDIAN should be defined!
+#endif
+
 #if BUILD_IS_BIG_ENDIAN // this is from config.h, set by cmake
   #define SDL_BYTEORDER SDL_BIG_ENDIAN
 #else

sys/sys_public.h

@@ -58,6 +58,7 @@ typedef enum {
 	SE_KEY,					// evValue is a key code, evValue2 is the down flag
 	SE_CHAR,				// evValue is an ascii char
 	SE_MOUSE,				// evValue and evValue2 are reletive signed x / y moves
+	SE_MOUSE_ABS,			// evValue and evValue2 are absolute x / y coordinates in the window
 	SE_JOYSTICK_AXIS,		// evValue is an axis number and evValue2 is the current state (-127 to 127)
 	SE_CONSOLE				// evPtr is a char*, from typing something at a non-game console
 } sysEventType_t;