Two pixelpipe bugfixes plus maintenance

[jenshannoschwalm]

(1) Bugfix CPU pixelpipe cache integrity after module colorspace conversion

Before a piece is processed the pixelpipe checks if the current cst is different from the module's requested input cst.
In this case, the input data are converted in-place (for less pixelpipe mem pressure) but then the cacheline - possibly kept for reuse - has an incorrect cst!

Note: in opencl code path this is different as the required colorspace conversion is always done in cl_mem and thus does not affect the pixelpipe cache.

For now we just invalidate that single cacheline.

Comment: Quite an old bug! Very hard to trigger with current code - might have been the underlying cause for some "fixes" done invalidating the pipecache :-) Also not sure if this could be a reason for false color issues while being in darkroom.

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(2) Performance regression: Fix preview pixelpipe caching issue

If we process the preview pixelpipe on CPU - either because there is no OpenCL device at all or the OpenCL scheduler delivered the pipe to the CPU - the importance hint for the cacheline was not tested correctly so with a performance penalty.

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(3) maintenance:The colorspace transform functions use dt_iop_colorspace_type_t()

Use correct dt_iop_colorspace_type_t instead of int for dt_ioppr_transform_image_colorspace() and dt_ioppr_transform_image_colorspace_cl()

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(4) Some maintenance work for pixelpipe processing

1. refactored _is_debug_pipe() also testing for a no-shutdown situation.
2. refactored the cpu bound benchmark loop for readability. Both benchmarks check for shutdown in (1)
3. some _pipe_has_shutdown() tests have been removed as clearly not helping
4. Some constify work, use of #define and log improvements

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@TurboGit no integration tests regressions, nothing "new"

[TurboGit]

Looks good, thanks!