Fix Buttons not working on Linux with newer gcc

The button mask is created by shifting a bit according to the
MouseCode, which is just a renamed ButtonCode_t.
Mouse buttons start at 107, which is way out of range for our ints.

To fix this, introduce MouseButtonBit(), which checks if a button code
corresponds to a mouse button at all and returns a usable bitmask
for the corresponding mouse button code.
This is then used for the button mask.

sp/src/public/vgui/MouseCode.h

@@ -18,6 +18,15 @@
 namespace vgui
 {
 typedef ButtonCode_t MouseCode;
+
+static inline int MouseButtonBit(MouseCode code)
+{
+	if (code < MOUSE_FIRST || code > MOUSE_LAST) {
+		Assert(false);
+		return 0;
+	}
+	return 1 << (code - MOUSE_FIRST);
+}
 }
 
 #endif // MOUSECODE_H

sp/src/vgui2/vgui_controls/Button.cpp

@@ -695,12 +695,12 @@ void Button::SetMouseClickEnabled(MouseCode code,bool state)
 	if(state)
 	{
 		//set bit to 1
-		_mouseClickMask|=1<<((int)(code+1));
+		_mouseClickMask|=MouseButtonBit(code);
 	}
 	else
 	{
 		//set bit to 0
-		_mouseClickMask&=~(1<<((int)(code+1)));
+		_mouseClickMask&=~MouseButtonBit(code);
 	}	
 }
 
@@ -709,7 +709,7 @@ void Button::SetMouseClickEnabled(MouseCode code,bool state)
 //-----------------------------------------------------------------------------
 bool Button::IsMouseClickEnabled(MouseCode code)
 {
-	if(_mouseClickMask&(1<<((int)(code+1))))
+	if(_mouseClickMask&MouseButtonBit(code))
 	{
 		return true;
 	}