Modify the logic of selection of overviews for non-nearest resampling…

[rouault]

…; add a GDAL_OVERVIEW_OVERSAMPLING_THRESHOLD config option

The current logic reads:

 * Some formats may efficiently implement decimation into a buffer by
 * reading from lower resolution overview images. The logic of the default
 * implementation in the base class GDALRasterBand is the following one. It
 * computes a target_downscaling_factor from the window of interest and buffer
 * size which is min(nXSize/nBufXSize, nYSize/nBufYSize).
 * It then walks through overviews and will select the first one whose
 * downscaling factor is greater than target_downscaling_factor / 1.2.
 *
 * Let's assume we have overviews at downscaling factors 2, 4 and 8.
 * The relationship between target_downscaling_factor and the select overview
 * level is the following one:
 *
 * target_downscaling_factor  | selected_overview
 * -------------------------  | -----------------
 * ]0,       2 / 1.2]         | full resolution band
 * ]2 / 1.2, 4 / 1.2]         | 2x downsampled band
 * ]4 / 1.2, 8 / 1.2]         | 4x downsampled band
 * ]8 / 1.2, infinity[        | 8x downsampled band

With this PR, is is ammended with the following complement:

 * Note that starting with GDAL 3.9, this 1.2 oversampling factor can be
 * modified by setting the GDAL_OVERVIEW_OVERSAMPLING_THRESHOLD configuration
 * option. Also note that starting with GDAL 3.9, when the resampling algorithm
 * specified in psExtraArg->eResampleAlg is different from GRIORA_NearestNeighbour,
 * this oversampling threshold defaults to 1. Consequently if there are overviews
 * of downscaling factor 2, 4 and 8, and that the desired downscaling factor is
 * 7.99, the overview of factor 4 will be selected for a non nearest resampling.

[coveralls]

coverage: 68.736% (-0.005%) from 68.741%
when pulling 9004633 on rouault:GDAL_OVERVIEW_OVERSAMPLING_THRESHOLD
into ccb472a on OSGeo:master.

[tbonfort]

I am seeing qgis doing some back and forth between the full-resolution data and the first overview, while only doing a small pan. I believe this is due to some numerical instability when calculating the downscaling factor, and the default GDAL_OVERVIEW_OVERSAMPLING_THRESHOLD=1 used.
To reproduce:

• open a raster in qgis
• right click on raster layer, select "zoom to native resolution"
• zoom out one level (one click on the mouse wheel)
• => when panning, qgis switches ,seemingly randomly, between the first overview level or the full resolution level.

This can be "fixed" by setting GDAL_OVERVIEW_OVERSAMPLING_THRESHOLD=1.01. I believe that the default value used for GDAL_OVERVIEW_OVERSAMPLING_THRESHOLD should be set to 1+epsilon in order to avoid this instability on commonly used zoom levels.

[rouault]

I am seeing qgis doing some back and forth between the full-resolution data and the first overview

should be fixed per #9100