[raster] Implement a version of rt_raster_deserialize for the raster header only

[pracine]

To get a much better performance on every ST_Get function on the raster header:

RASTER_convex_hull[[BR] RASTER_getSRID[[BR] RASTER_getWidth[[BR] RASTER_getHeight[[BR] RASTER_getNumBands[[BR] RASTER_getXScale[[BR] RASTER_getYScale[[BR] RASTER_setScale[[BR] RASTER_setScaleXY[[BR] RASTER_getXSkew[[BR] RASTER_getYSkew[[BR] RASTER_setSkew[[BR] RASTER_setSkewXY[[BR] RASTER_getXUpperLeft[[BR] RASTER_getYUpperLeft[[BR] RASTER_setUpperLeftXY

See if it is possible to do the same for a requested band so we can improve the performance of every other functions.