Survey map fixes

[tritonas00]

Fixes #2767

For some reason when texture filtering is set to Bilinear, with some gpus, ImGui::GetIO().DisplaySize.y returns 0
Old code was: https://github.com/RigsOfRods/rigs-of-rods/blob/2020.01/source/main/terrain/map/SurveyMapManager.cpp#L185

Also fixes darker image in Big map with Anisotropic filtering

Before:

After:

@paroj any idea why those nasty things happen? 🤣

[ohlidalp]

@paroj I think this problem may affect Ogre as well since our imgui integration is a port of imgui-overlay from Ogre 1.12

This is where we fill DisplaySize:

rigs-of-rods/source/main/gui/imgui/OgreImGuiOverlay.cpp

Lines 139 to 140 in c5ee81a

	// Setup display size (every frame to accommodate for window resizing)
	io.DisplaySize = ImVec2(oMgr.getViewportWidth(), oMgr.getViewportHeight());

Ogre 13's code is equivalent: https://github.com/OGRECave/ogre/blob/493578624d6cf5513983b1df49fa945dd9cee26f/Components/Overlay/src/OgreImGuiOverlay.cpp#L146-L147

[paroj]

but the NewFrame() signature and calling convention changed upstream