The gdal/swig/python/samples/ogr_layer_algebra.py should be promoted to an official Python script

Steps:

• move it to gdal/swig/python/scripts
• create a new test script for it in autotest/pyscripts/test_ogr_layer_algebra.py
• write a documentation page in doc/source/programs/ogr_layer_algebra (and link it to the index)

@elrnv pointed out in #823 that CooMatrix could benefit from a reserve function similar to Vec::reserve for reserving space for additional triplets.

When I edit a polygon with a zoom level low and I snap this polygon to another one, it's very common the created snap point is not exactly on the line / point.
For me, it's a problem regarding I control if these polygons are not intersecting.

Is your feature request related to a problem? Please describe.
Can't use keyboard mnemonics Alt + O or Alt + C to apply or cancel changes

Describe the solution you'd like
Implement the mnemonics

Describe alternatives you've considered
Use mouse click to apply or cancel changes

Additional context
![Screenshot from 2021-02-21 13-29-19](https://user-images.githubuserc

Add Runnable links of the code below the example.
Use link name Run Code which links to a Online C++ complier conatining the example code.
Use Rextester as the Online C++ Compiler (If you have a better suggesstion please comment).

Follow below steps to generate the Run Code Link.

1. Go to Rextester
2. Copy or write the example

An option like $ fio cat --bounds w s e n --bounds-crs <crs> --dst-crs <crs> would allow for filtering prior to reprojection, like $ ogr2ogr -spat w s e n -spat_srs <crs>.

See Toblerity/Fiona#210 for a discussion about standardizing bounding boxes as --bounds w s e n rather than --bbox w,s,e,n.