BGE : Fix T36285 Wrong viewport scale.

Fix of T36285, I just invalidate projection matrix when a camera switch
to viewport mode, because we need to rewrite the projection matrix in
RenderFrame.
It worked with old blender version because when you use viewport render
one camera is added in the camera list and initialize its projection
matrix for the first time. But when we used several cameras + viewport
for splitscreen we had issues.

Reviewers: dfelinto, hg1, ben2610, lordloki, moguri

Reviewed By: moguri

Projects: #game_rendering, #game_engine

Differential Revision: https://developer.blender.org/D1202

source/gameengine/Ketsji/KX_Camera.cpp

@@ -463,6 +463,7 @@ bool KX_Camera::GetFrustumCulling() const
  
 void KX_Camera::EnableViewport(bool viewport)
 {
+	InvalidateProjectionMatrix(false); // We need to reset projection matrix
 	m_camdata.m_viewport = viewport;
 }