gdi32: add test for SelectClipRgn in metafiles, make it pass

[Dan Kegel]

Adding a simple SelectClipRgn metafile test revealed bugs in
SelectClipRgn metafile support:

- in MFDRV_CreateRegion():
   - one length field was off by factor of two
   - another length field was off by four
   - forgot to finish generating final band
   - The reserved "objectCount" field didn't have the same value as on Windows

- in MFDRV_ExtSelectClipRgn, generate META_SELECTOBJECT like Windows
does, not META_SELECTCLIPREGION.

Fixing the code to pass the test also fixes bug 30923.

The patch also adds a couple comments and moves a couple misplaced test calls.
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