Screenshoter: fix screenshot folder

[hius07]

Check for existance and create if absent.
Closes #12742.

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This change is 

[NiLuJe]

Oops, skipped over the CI: frontend/ui/widget/screenshoter.lua:103:39: shadowing upvalue argument caller_callback on line 49 ;).

[hius07]

CircleCI error message

Found ninja-1.10.0 at /usr/bin/ninja
ninja: build stopped: subcommand failed.
make: *** [base/Makefile:89: base-all] Error 1

@benoit-pierre can you help with this?

[benoit-pierre]

sigh

Something changed on CircleCI's side, so we're trying to build with too many jobs again:

************ NINJA: ninja -j54 -l36 **********
************ MAKE: make -j54 -l36 **********

[hius07]

Dunno if it's related, a notice from them:

Notice: If you are using "when: always" in your workflow filters, update to "when: true" to avoid configuration failures.

[benoit-pierre]

Dunno if it's related, a notice from them:

Notice: If you are using "when: always" in your workflow filters, update to "when: true" to avoid configuration failures.

Confusing and misleading, this does not apply to our case (Cf. https://circleci.com/changelog/dynamic-when-statements-breaking-change/):

NOTE: When in Steps are not impacted by this change.

[NiLuJe]

Err, can it be anything else?

[hius07]

Yes, ges_event sends a gesture object as the second arg.

[hius07]

koreader/frontend/ui/widget/screenshoter.lua

Lines 119 to 121 in 8f98a44

	Screenshoter.onKeyPressShoot = Screenshoter.onScreenshot
	Screenshoter.onTapDiagonal = Screenshoter.onScreenshot
	Screenshoter.onSwipeDiagonal = Screenshoter.onScreenshot