Fix: Handle HGT void values (-32768) in bilinear and nearestNeighbour

[MariuszTurek-W3media]

Fixes #33

HGT elevation files contain voids (no-data) flagged with the value -32768. Previously, these values were included in interpolation calculations, producing incorrect elevation results (e.g. -10265 instead of 459).

Changes

• Hgt.bilinear: Skip void samples when interpolating. If one neighbour in a pair is void, use the other. If both pairs are void on one axis, fall back to the other axis. Returns null only when all four surrounding samples are voids.
• Hgt.nearestNeighbour: Return null instead of -32768 when the nearest sample is a void.
• Added HGT_VOID constant (-32768) for clarity.

Examples

All four neighbours are voids:
Lng 20.433394, Lat 49.408558

v00	v10	v11	v01
-32768	-32768	-32768	-32768

Returns null

Two valid neighbours, two voids:
Lng 20.432394, Lat 49.408558

v00	v10	v11	v01
461	-32768	-32768	493

Returns 469.63