iidx: workaround for touch monitors that report contact area (#462)

## Link to GitHub Issue, if one exists
n/a

## Description of change
When touch monitors send touch area size information
(`TOUCHEVENTFMASK_CONTACTAREA` flag and `cxContact` / `cyContact` fields
of `TOUCHINPUT` struct) from `GetTouchInputInfo`, IIDX gets confused and
does some unexpected things when interacting with the subscreen.

As a workaround, hook `GetTouchInputInfo` and clear out the flag &
values.

This is only done for IIDX AND when -iidxnativetouch is on.

## Testing
Tested on a Dell touch monitor, simulating "bad" values.

src/spice2x/CMakeLists.txt

@@ -526,6 +526,7 @@ set(SOURCE_FILES ${SOURCE_FILES}
         misc/device.cpp
         misc/eamuse.cpp
         misc/extdev.cpp
+        misc/nativetouchhook.cpp
         misc/sciunit.cpp
         misc/sde.cpp
         misc/wintouchemu.cpp