Fix linear flip.left.x / flip.right.x rendering on desktop pygame/SDL

[foonerd]

Summary

• Parse flip.left.x / flip.right.x as real booleans (aligned with left.needle.flip / right.needle.flip), so False is no longer treated as enabled.
• When flipping a linear indicator, copy the surface first (channels often share one cached image), then convert_alpha() after transform.flip so the flipped indicator remains blit-safe on desktop pygame/SDL backends.
• Initialize linear mask source rectangles in image-local coordinates so the first draw does not sample outside the indicator image.

Why

Horizontal flip for linear meters is applied in LinearAnimator. On some pygame/SDL desktop configurations the flipped surface fails to blit useful pixels (indicator looks missing or unflipped). Stereo meters also commonly reuse one cached indicator Surface for left and right; flipping without a detached copy is unsafe when both channels need independent content. Separately, reset_mask previously stored screen position in the blit source rect, which can yield an empty first paint.

Benefit

• flip.left.x / flip.right.x work reliably for linear meters across embedded and desktop pygame/SDL hosts.
• flip.*.x = False means off (previously the non-empty string "False" was truthy).
• Stock meters that use flip (relax, steam-punk, fantasy, and custom themes) render as designed.

Impact

Area	Impact
Storage / flash wear	Negligible. No extra files on disk. At most one extra in-memory surface copy per flipped channel when a linear meter starts (not per frame).
Performance	Negligible ongoing cost. Flip runs once in LinearAnimator.__init__ (meter start / theme change). Extra work is Surface.copy + transform.flip + optional convert_alpha per flipped channel. No change to the per-frame run() path.
Footprint (RAM)	Small and temporary relative to meter assets: one additional full-size indicator surface per flipped channel for the life of that meter instance (same order as the indicator PNG already loaded). Unflipped meters unchanged.
Config compatibility	Additive / corrective. Existing True keeps working. Explicit False now correctly disables flip. Missing key remains off.

Files

• configfileparser.py - boolean parse helper for flip keys
• linear.py - detached flip + convert_alpha
• meter.py - default flip flags; image-local reset_mask

[foonerd]

@project-owner, @Gjuju - FYI

[project-owner]

@foonerd, thank you for the changes! Just one comment, let's use the current naming convention and avoid underscore as a function name prefix (e.g. _get_optional_boolean etc). Thanks!

[foonerd]

@project-owner Thanks for the review, and for catching the naming.

The leading underscores were only a Python “private helper” habit from my side (_get_optional_boolean, _as_bool, _flip_x), not meant as a PeppyMeter convention. I’ve dropped them to match the existing style: get_optional_boolean, as_bool, flip_x. Behavior is unchanged.