Handle MultiPoints in DXF writer (fixes #11686)

[michkowalczuk]

What does this PR do?

Adds support for writing MultiPoints by DXF driver.
#11686

What are related issues/pull requests?

It comes from this thread.

Tasklist

• ADD YOUR TASKS HERE
• Make sure code is correctly formatted (cf pre-commit configuration)
• Add test case(s)
• Add documentation
• Updated Python API documentation (swig/include/python/docs/)
• Review
• Adjust for comments
• All CI builds and checks have passed

Environment

Provide environment details, if relevant:

• OS:
• Compiler:

[coveralls]

coverage: 70.06% (-0.003%) from 70.063%
when pulling e1b5dd3 on michkowalczuk:handle-MultiPoints-in-DXF-writer
into 3da6544 on OSGeo:master.

[rouault]

CC @atlight . Looks reasonable to me. A new autotest could be appropriate, but I could live without it too here.

[michkowalczuk]

CC @atlight . Looks reasonable to me. A new autotest could be appropriate, but I could live without it too here.

@rouault Can I just update "test_ogr_dxf_12" test function in ogr_dxf.py? Or create a new test definition?

[rouault]

Can I just update "test_ogr_dxf_12" test function in ogr_dxf.py? Or create a new test definition?

that would be fine. Change the comment of the test to "Write a simple file with a few geometries of different types, and read back"