Reset Pointer.buttons to 0 on mouseup

[SonnyCampbell]

Please do not update the README or Change Log, we will do this when we merge your PR.

This PR (delete as applicable)

• Fixes a bug

Describe the changes below:
The leftButtonDown(), rightButtonDown(), etc functions currently don't return false after releasing the button on the mouse. There's a check in Pointer.up() for:

if (event.buttons) { this.buttons = event.buttons; }

However event.buttons is always 0 on mouseup, so this.buttons never gets reset to 0.

[xRahman]

Hello,

there is still one problem even after applying your fix.

If you press a button (inside canvas), move the mouse out of canvas, release that button and move the mouse back into the canvas, button is reporeted in pointer events as 'down' even though it's not down anymore.

[rexrainbow]

@xRahman Related issue: #3779

[photonstorm]

Thanks for this PR. I've implemented a fix into the Pointer class locally which resolves this issue, so although I'm not going to merge the PR, the fix is in and attributed to you in the change log.