tech-overview.txt: reflect talloc/TA changes
x11_common: don't handle mouse enter events

This could cause the OSC to be displayed without mouse interaction: for
example, starting mpv with --fs, and putting the mouse to where the OSC
area is beforehand, would cause the OSC to appear and stay visible. We
don't want that. The simplest solution is not generating artificial
mouse move events from mouse enter events, because they make the OSC
think the mouse was actually moved.

Also see commit 0c7978c, where handling of mouse enter events was added.
This was supposed to fix certain corner cases, but they're not relevant
anymore due to changes in OSC behavior.

Commit 9777047 fixed this as well (by resetting the mouse state on
MOUSE_LEAVE), but all the behavior reverted with this commit as perhaps
a bad idea. It wasn't very robust, made it hard to distinguish real
events from artificial ones, and finally made the mouse cursor more
often visible than needed. (Now switching between workspaces doesn't
make the cursor visible again when switching to a fullscreened mpv.)