Use stricter typing for void* pTransformerArg

[rouault]

@elpaso Implements what you discussed in #10952 (comment)

The last commit ("Make GDALTransformerArg a typedef of an anonymous struct GDALTransformerArgOpaque*") is slightly backwards incompatible

[coveralls]

coverage: 69.434%. remained the same
when pulling b9d0adb on rouault:GDALTransformerArg
into 15b4921 on OSGeo:master.

[rouault]

The last commit ("Make GDALTransformerArg a typedef of an anonymous struct GDALTransformerArgOpaque*") is slightly backwards incompatible

We might also postpone this to 3.11, as there's no emergency in merging this, especially if @dbaston would want a cleaner way of doing proper C++ typing.

[github-actions]

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