vgui_support: remove utl dependency, by porting new vgui support code from original engine, remove dead function definitions, remove dead vgui_main.h header from engine

vgui_support/vgui_clip.cpp

@@ -43,8 +43,8 @@ void EnableScissor( qboolean enable )
 void SetScissorRect( int left, int top, int right, int bottom )
 {
 	// Check for a valid rectangle...
-	Assert( left <= right );
-	Assert( top <= bottom );
+	assert( left <= right );
+	assert( top <= bottom );
 
 	g_ScissorRect.left = left;
 	g_ScissorRect.top = top;
@@ -121,4 +121,4 @@ qboolean ClipRect( const vpoint_t &inUL, const vpoint_t &inLR, vpoint_t *pOutUL,
 	}
 	return true;
 }
-}
+}