More OpenGL fixes plus masked PNG conversion

See merge request !619

Fix mouse being grabbed even when not used

See merge request !639

The other way around this time.

This reverts commit 6c85c4e1.

Fix usage of V_GetColor

See merge request !638

Use GCC 8.1 for AppVeyor 64-bit builds

See merge request !624

Add clipboard functionality to the Connect via IP textbox.

See merge request !597

I was using V_GetColor in a lot of places I shouldn't have, making the game look wrong with a non-default colour profile.
Though, I left R_RainbowColormap alone.

Fix major issue

See merge request !625

Remove Direct Draw from AppVeyor config

See merge request !622

Do not save netgame-synced console variables

See merge request !623

Let the mouse move freely when a menu is open or game is paused

See merge request !617

This is a bad thing to do, because if you join a server,
your game will save the host's settings.

This reverts commit 17636ccc.

NOHW V.S. Renderer Switching

See merge request !609

Stuff I Broke: Commands

See merge request !607

Properly fix the issue that !528 tried to; let the -warp parameter work with map name

See merge request !596

That means you can now easily move your mouse out of SRB2's window
and switch between several windows easily by just pressing escape!

Any phase of the game that isn't actual gameplay counts as a menu,
which means you can also move the mouse in cutscenes,
at the title screen, server connection screen,
and even when the chat or console are open.

Revert "Merge branch 'software-clownery' into 'master'"

See merge request !616