fix(GFX_Button): initialize currstate/laststate

[94xhn]

Fixes #401.

Problem

Adafruit_GFX_Button's constructor only initializes _gfx:

Adafruit_GFX_Button::Adafruit_GFX_Button(void) { _gfx = 0; }

currstate/laststate (declared in Adafruit_GFX.h with no default member initializer) are left as indeterminate memory. justPressed()/justReleased() read them:

bool Adafruit_GFX_Button::justPressed() { return (currstate && !laststate); }
bool Adafruit_GFX_Button::justReleased() { return (!currstate && laststate); }

If either is called before the first press() call, the result depends on whatever garbage happened to be on the stack/heap at construction time - a button that was never touched can spuriously report a press or release transition.

Fix

Initialize both to false in the constructor, matching the "not pressed, no prior state" starting condition every other code path already assumes.

Test plan

No hardware handy, so I verified the logic with a standalone harness modeling the class's exact fields/methods: before the fix, constructing a Button over deliberately non-zeroed memory and calling justPressed() before any press() call can return true; after the fix, it correctly returns false.

Disclosure

Generative AI (Claude) was used to help investigate this issue, implement, and verify this fix. All changes were reviewed by me before submission.