Support grabbing the pointer with the Pointer Lock API

[kasmtech]

This change adds the following:

a) A new button on the UI to enter full pointer lock mode, which invokes the Pointer Lock API[1] on the canvas, which hides the cursor and makes mouse events provide relative motion from the previous event (through movementX and movementY). These can be added to the previously-known mouse position to convert it back to an absolute position. b) Adds support for the VMware Cursor Position pseudo-encoding[2], which servers can use when they make cursor position changes themselves. This is done by some APIs like SDL, when they detect that the client does not support relative mouse movement[3] and then "warp"[4] the cursor to the center of the window, to calculate the relative mouse motion themselves. c) When the canvas is in pointer lock mode and the cursor is not being locally displayed, it updates the cursor position with the information that the server sends, since the actual position of the cursor does not matter locally anymore, since it's not visible. d) Adds some tests for the above.

You can try this out end-to-end with TigerVNC with TigerVNC/tigervnc#1198 applied!

Fixes: #1493 under some circumstances (at least all SDL games would now work).

1: https://developer.mozilla.org/en-US/docs/Web/API/Pointer_Lock_API 2: https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#vmware-cursor-position-pseudo-encoding 3: https://hg.libsdl.org/SDL/file/28e3b60e2131/src/events/SDL_mouse.c#l804 4: https://tronche.com/gui/x/xlib/input/XWarpPointer.html