5 button mouse support from b333rt in IRC with some edits for X11.

Tested in X11 where its fairly confusing.
buttons 4 and 5 are used for the wheel which is well known, but it seems 6 and 7 are used for horizontal scrolling, my mouse assigns the extra 2 buttons to events 8 & 9.

So the X11 events used for buttons called 4&5 in blender are 8&9 in X11.

The mouse buttons can be re-ordered like this once xorg starts (swaps 6,7 with 8,9)
  xmodmap -e "pointer = 1 2 3 4 5 8 9 6 7"

Couldn't test Win32, Apple not supported.
If someone wants to add horizontal scrolling its quite easy.

3 files changed, 41 insertions, 2 deletions

diff --git a/intern/ghost/GHOST_Types.h b/intern/ghost/GHOST_Types.h
index 73ed0bdd1fa..31819f341a0 100644
--- a/intern/ghost/GHOST_Types.h
+++ b/intern/ghost/GHOST_Types.h
@@ -132,6 +132,8 @@ typedef enum {
 	GHOST_kButtonMaskLeft = 0,
 	GHOST_kButtonMaskMiddle,
 	GHOST_kButtonMaskRight,
+	GHOST_kButtonMaskButton4,
+	GHOST_kButtonMaskButton5,
 	GHOST_kButtonNumMasks
 } GHOST_TButtonMask;
 
diff --git a/intern/ghost/intern/GHOST_SystemWin32.cpp b/intern/ghost/intern/GHOST_SystemWin32.cpp
index 8513d056795..2e89be40bcb 100644
--- a/intern/ghost/intern/GHOST_SystemWin32.cpp
+++ b/intern/ghost/intern/GHOST_SystemWin32.cpp
@@ -39,7 +39,6 @@
 #endif
 
 #include "GHOST_SystemWin32.h"
-//#include <stdio.h> //for printf()
 
 // win64 doesn't define GWL_USERDATA
 #ifdef WIN32
@@ -61,6 +60,23 @@
 #define WHEEL_DELTA 120	/* Value for rolling one detent, (old convention! MS changed it) */
 #endif // WHEEL_DELTA
 
+/* 
+ * Defines for mouse buttons 4 and 5 aka xbutton1 and xbutton2.
+ * MSDN: Declared in Winuser.h, include Windows.h 
+ * This does not seem to work with MinGW so we define our own here.
+ */
+#ifndef XBUTTON1
+#define XBUTTON1 0x0001
+#endif // XBUTTON1
+#ifndef XBUTTON2
+#define XBUTTON2 0x0002
+#endif // XBUTTON2
+#ifndef WM_XBUTTONUP
+#define WM_XBUTTONUP 524
+#endif // WM_XBUTTONUP
+#ifndef WM_XBUTTONDOWN
+#define WM_XBUTTONDOWN 523
+#endif // WM_XBUTTONDOWN
 
 #include "GHOST_Debug.h"
 #include "GHOST_DisplayManagerWin32.h"
@@ -672,6 +688,14 @@ LRESULT WINAPI GHOST_SystemWin32::s_wndProc(HWND hwnd, UINT msg, WPARAM wParam,
 					window->registerMouseClickEvent(true);
 					event = processButtonEvent(GHOST_kEventButtonDown, window, GHOST_kButtonMaskRight);
 					break;
+				case WM_XBUTTONDOWN:
+					window->registerMouseClickEvent(true);
+					if ((short) HIWORD(wParam) == XBUTTON1){
+						event = processButtonEvent(GHOST_kEventButtonDown, window, GHOST_kButtonMaskButton4);
+					}else if((short) HIWORD(wParam) == XBUTTON2){
+						event = processButtonEvent(GHOST_kEventButtonDown, window, GHOST_kButtonMaskButton5);
+					}
+					break;
 				case WM_LBUTTONUP:
 					window->registerMouseClickEvent(false);
 					event = processButtonEvent(GHOST_kEventButtonUp, window, GHOST_kButtonMaskLeft);
@@ -684,6 +708,14 @@ LRESULT WINAPI GHOST_SystemWin32::s_wndProc(HWND hwnd, UINT msg, WPARAM wParam,
 					window->registerMouseClickEvent(false);
 					event = processButtonEvent(GHOST_kEventButtonUp, window, GHOST_kButtonMaskRight);
 					break;
+				case WM_XBUTTONUP:
+					window->registerMouseClickEvent(false);
+					if ((short) HIWORD(wParam) == XBUTTON1){
+						event = processButtonEvent(GHOST_kEventButtonUp, window, GHOST_kButtonMaskButton4);
+					}else if((short) HIWORD(wParam) == XBUTTON2){
+						event = processButtonEvent(GHOST_kEventButtonUp, window, GHOST_kButtonMaskButton5);
+					}
+					break;
 				case WM_MOUSEMOVE:
 					event = processCursorEvent(GHOST_kEventCursorMove, window);
 					break;
diff --git a/intern/ghost/intern/GHOST_SystemX11.cpp b/intern/ghost/intern/GHOST_SystemX11.cpp
index 9f6f3b4d5b0..cdbdce9c2ca 100644
--- a/intern/ghost/intern/GHOST_SystemX11.cpp
+++ b/intern/ghost/intern/GHOST_SystemX11.cpp
@@ -444,10 +444,15 @@ GHOST_SystemX11::processEvent(XEvent *xe)
 
 			XButtonEvent & xbe = xe->xbutton;
 			GHOST_TButtonMask gbmask = GHOST_kButtonMaskLeft;
-
 			switch (xbe.button) {
 				case Button1 : gbmask = GHOST_kButtonMaskLeft; break;
 				case Button3 : gbmask = GHOST_kButtonMaskRight; break;
+				/* It seems events 6 and 7 are for horizontal scrolling.
+				 * you can re-order button mapping like this... (swaps 6,7 with 8,9)
+				 *   xmodmap -e "pointer = 1 2 3 4 5 8 9 6 7" 
+				 */
+				case 8 : gbmask = GHOST_kButtonMaskButton4; break; /* Button4 is the wheel */
+				case 9 : gbmask = GHOST_kButtonMaskButton5; break; /* Button5 is a wheel too */
 				default:
 				case Button2 : gbmask = GHOST_kButtonMaskMiddle; break;
 			}