Support grabbing the pointer with the Pointer Lock API

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: novnc#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