fix(xbilparser): apply zmin / zmax for any texture subsampling size

[airnez]

Description

Min and max clamping elevation parameters were not applied when subsampling parent tile elevation texture with a window smaller that two pixels wide.

This implies that some tiles are not clamped when applying a strong zoom, like switching from the globe view to a city without animation, using STRATEGY_MIN_NETWORK_TRAFFIC. For a custom terrain, setting the usual -99999 nodata value and clamping minimum altitude to 0 results in unexpected culling removing the terrain. See the screenshot.

After looking at git history, I suppose that this might just be a coding mistake, putting these conditions in the wrong code block. What do you think @gchoqueux ?

Screenshots (if appropriate)

zoom-zmin-terrain.mp4