Improve relativeToVRT sources management in VRTRawRasterBands

[avalentino]

The path addresses some inconsistencies relative to the SourceFilename of VRTRawRasterBands when the relativeToVRT element is set to ture.

With the current implementation datasets that are successfully created in a sub-directory can't be reopened for reading:

import os
import numpy as np
from osgeo import gdal
from osgeo.gdal_array import NumericTypeCodeToGDALTypeCode

gdal.UseExceptions()

filename = 'subdir/data.vrt'
os.makedirs('subdir', exist_ok=True)
data = np.ones((10, 10))
drv = gdal.GetDriverByName('VRT')
ysize, xsize = data.shape
gdtype = NumericTypeCodeToGDALTypeCode(data.dtype)
ds = drv.Create(filename, xsize, ysize, 0)
srcpath = os.path.splitext(filename)[0] + '.dat'
pixel_offset = gdal.GetDataTypeSize(gdtype) // 8
line_offset = xsize * pixel_offset
options = {
    'subClass': 'VRTRawRasterBand',
    'SourceFilename': srcpath,
    'ImageOffset': 0,
    'PixelOffset': pixel_offset,
    'LineOffset': line_offset,
    'relativeToVRT': 1,
}
options = ['{}={}'.format(k, v) for k, v in options.items()]
ds.AddBand(gdtype, options)
b = ds.GetRasterBand(1)
b.WriteArray(data)
ds.FlushCache()

data2 = gdal.Open(filename).ReadAsArray()

the above script gives the following error:

RuntimeError                              Traceback (most recent call last)
<ipython-input-1-84f5e1183627> in <module>()
     30 ds.FlushCache()
     31 
---> 32 data2 = gdal.Open(filename).ReadAsArray()
     33 

RuntimeError: Unable to open subdir/subdir/data.dat.
No such file or directory

[rouault]

Committed in trunk r29691