Add CPLErrorOnce() and CPLDebugOnce()

[rouault]

to emit an error/debug message only once during the life-time of a process.

[coveralls]

coverage: 70.075% (-0.001%) from 70.076%
when pulling fb37e01 on rouault:CPLErrorOnce
into 1d3a440 on OSGeo:master.

[sgillies]

@rouault like Python warnings, then? Use it in code as many times as we want, but a unique error message/code combination appears no more than once?

[rouault]

like Python warnings, then? Use it in code as many times as we want, but a unique error message/code combination appears no more than once?

yes, this just institutionalizes a practice that has been done at hand up to now. Some GDAL errors/warnings are sometimes in tight loops or would be too frequently emitted.

[dbaston]

This is a nice addition. I had a couple of ideas, please feel free to disregard:

• should we allow the warning to be raised a small N number of times? Since the implementation isn't testing the uniqueness of the message I can imagine cases where seeing a message like "Value %d for field %s does not fit into a short and will be clamped." with a few different values of %d and %s could make it easier to understand the source of the error.
• should we automatically append something like "Further messages of this type will be suppressed" to the text?

[rouault]

• should we allow the warning to be raised a small N number of times?

I can see the point, but dont' feel like going into that complication currently.

should we automatically append something like "Further messages of this type will be suppressed" to the text?

good idea, done