RFR: 8303038: Glass gtk3 sends scroll events with delta(x, y) = 0 [v7]

[Thiago Milczarek Sayao]

> Simple fix to get the scroll deltas from GDK_SCROLL_SMOOTH. If we ignore this scroll event type, deltas are sent to java with the value equal to zero.
> 
> Here's whats happening:
> 
> We include all event masks, so when using gtk3 (>= 3.4.0) it includes `GDK_SMOOTH_SCROLL_MASK` meaning we receive duplicated events, one with  `direction = GDK_SMOOTH_SCROLL_MASK` and other with "legacy" direction (UP/DOWN).
> 
> When receiving the event corresponding to `GDK_SMOOTH_SCROLL_MASK` we ignored it causing it to send deltas (x,y) = 0.
> 
> The fix now checks if `GDK_SMOOTH_SCROLL_MASK` is supported and uses it, also adding smooth scroll functionality.

Thiago Milczarek Sayao has updated the pull request incrementally with one additional commit since the last revision:

  Remove print statement

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1044/files
  - new: https://git.openjdk.org/jfx/pull/1044/files/80285aef..15423b0c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1044&range=06
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1044&range=05-06

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1044.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1044/head:pull/1044

PR: https://git.openjdk.org/jfx/pull/1044