steamcompmgr: Add env variable to force disable cursor grabbing

This is very convenient for playing visual novels in windowed mode.

src/steamcompmgr.cpp

@@ -1887,7 +1887,7 @@ bool MouseCursor::getTexture()
 	m_texture = nullptr;
 
 	// Assume the cursor is fully translucent unless proven otherwise.
-	bool bNoCursor = true;
+	bool bNoCursor = !getenv( "GAMESCOPE_FORCE_CURSOR" );
 
 	std::shared_ptr<std::vector<uint32_t>> cursorBuffer = nullptr;