Use native QGIS raster API instead of GDAL API in zonal statistics

[alexbruy]

This should allow users to use zonal statistics with rasters which come not only from GDAL provider but also from other providers, e.g. WCS. Related issue https://hub.qgis.org/issues/8736

[Gustry]

Thanks @alexbruy and @wonder-sk ;-)

[wonder-sk]

@Gustry all credit goes to Alex, I have not done anything to implement this :-)

[Gustry]

Ahah, I though it was with your new feature to write a raster from the QGIS api without using gdal

[nyalldawson]

Very nice!

Just for my own piece of mind... do you mind timing some zonal stats operations before and after this change? Just want to make sure there's no speed regression here.

[alexbruy]

@nyalldawson I did quick tests using NOAAs temperature data and 2013 TIGER counties file (3234 features). Here is average result from 3 runs for each version

Using GDAL	QGIS API (debug output)	QGIS API (no debug output)
24590.3333	28579.3333	24591.2586

QGIS API (debug output) slower because readBlock() method in qgsgdalprovider.cpp produces some debug strings (around 5 lines for each call). If these debug strings suppressed, it shows almost the same result as GDAL version.

[nyalldawson]

Kill the debug code!

That's encouraging results. Another benefit I realised from this is that it means zonal stats will correctly handle rasters with an offset or scale.

+1 to merge

[nyalldawson]

@alexbruy this all looks great to me - I say merge away!