Passing custom parameters to gdal_merge

[HueyNemud]

Were're calling gdal-merge with a fixed set of options :
parameters = [ "", "-o", ofile, "-co", "COMPRESS=LZW", ]
We can give the user more control over the process by letting him pass a list of parameters for gdal_merge, for instance as an optional string :
scrapwms <layer> <output> --gdal-merge "-co COMPRESS=JPEG -of Gtiff"

⚠️ it would create some side effects :

• users will be able to change the format of the outputed regions with the option -of. Therefore, we would need to change the file extension accordingly.
• same goes for the name of the output file.