Put tablet input emulation behind an environment variable

Emulating tablet input via pointer input creates a few problems:

- the first one is that the same input event can be processed several
  times. The first time as a tablet tool event, the second time as a
  pointer event. Such a behavior can be unexpected and it did causes
  issues in the past with the input in decorations, for example
- the second one is the overall impact on the code. It increases the
  coupling between independent abstractions, such as tablet and
  pointer input, and it makes the code far more difficult to adapt
  to new design principles
- the third one is that it brings a new category of challenges related
  to managing the wl_pointer state when the tablet tool interferes
  with its state. This increases the potential for bugs where input
  misbehaves or just plain crashes

On the other hand, all major clients and toolkits have already added
support for the zwp_tablet_v2 protocol. In addition to that, another
major wayland compositor, mutter, doesn't even attempt to simulate
tablet tool events, and mutter developers have confirmed that they have
not received much complaints about this.

Emulating tablet input was necessary when the tablet input protocol
was still young and most clients didn't provide support for it, but as
it has matured, I believe it's not needed anymore.

As the first step, this change disables tablet input emulation by
default. In order to enable tablet input emulation, the
KWIN_WAYLAND_EMULATE_TABLET=1 environment variable must be set. Note
that tablet input emulation can be buggy because of the changes that
went in to prepare the code for fixing tablet tool event filters.
Tablet input emulation is expected to be dropped a couple of releases
later, which hopefully, will provide enough of time for the clients
that still don't support tablet input to add support for it.

src/input.cpp

@@ -2385,7 +2385,9 @@ public:
 
     bool emulateTabletEvent(TabletEvent *event)
     {
-        if (!workspace()) {
+        // Tablet input emulation is deprecated. It will be removed in the near future.
+        static bool emulateInput = qEnvironmentVariableIntValue("KWIN_WAYLAND_EMULATE_TABLET") == 1;
+        if (!emulateInput) {
             return false;
         }