touch: restore wintouchemu, fall back to it when native touch hooks fail (#834)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #833

## Description of change
Last couple PRs - such as #820 #827 #828 - made the native touch hook &
touch injection using `InjectTouchInput` the default path, since it
performs much more reliably with both real touch screens and mouse (or
any other synthetic source).

However, user has reported that WINE lacks `InjectTouchInput` which
means this won't work.

As a fix, revive the old wintouchemu code. Native touch is still the
default, but under following circumstances:

1. if `-touchemuforce` is set, or
2. if any of the required Windows touch APIs are unavailable

then we fail over from native touch to wintouchemu code. 

For Linux, condition #2 would be hit during init, and gracefully switch
over.

Caveat: the poke code for IIDX and Nost will continue to require native
touch, I do not want to maintain two paths for this. This means that
iidx poke will stop working on Linux, unfortunately.

## Testing
Tested on Windows with `-touchemuforce` set. This is mostly reverting
Linux code path back to where we were last release, so this should just
work with wine.

src/spice2x/touch/native/inject_synthetic.cpp

@@ -113,11 +113,12 @@ namespace nativetouch::inject {
     }
 
     static HWND prepare_synthetic_touch() {
-        initialize_touch_injection();
+        if (!initialize_touch_injection()) {
+            return nullptr;
+        }
 
         const auto window = get_injection_window();
-        if (!touch_injection_available() || window == nullptr ||
-            synthetic_touch_message == 0) {
+        if (window == nullptr || synthetic_touch_message == 0) {
             return nullptr;
         }
         return window;