opencl: fix a few syntactical glitches
opencl: make it a configuration option whether to synch opencl buffers with pixelpipe cache

for slow GPUs this makes sense, as reprocessing the whole pixelpipe is expensive
for fast GPUs the added overhead (PCIe memory transfers) slows down things dramatically

default is off
opencl: make micro naps during opencl processing a configuration option
opencl: avoid uncoalesced memory accesses in module nlmeans
opencl: avoid uncoalesced memory accesses in module denoiseprofile
opencl: non-blocking write to get image data from host to device
opencl: give configuration option on number of event handles
opencl: add configuration option to run GPU pipe asynchronously to pixelpipe
Merge branch 'master' into opencl
Revert "opencl: avoid uncoalesced memory accesses in module nlmeans"

This reverts commit c77838b0c1ecf760126c2edef070f00e5181de6b.

Was not working as expected and in fact runs slower than before.
Revert "opencl: avoid uncoalesced memory accesses in module denoiseprofile"

This reverts commit ef23e5a6f1fc7630138634d2701ab8af3f691a24.

Does not work as expected and in fact runs slower than before.
Merge branch 'master' into opencl
opencl: re-activate dt_opencl_finish() in loops of nlmeans/denoiseprofile

without this blocking synchronization point our pixelpipe consumes such a
big part of GPU cycles that screen updates get very jerky
Merge branch 'master' into opencl
opencl: nlmeans/denoiseprofile: use bucket chain of buffers for better latency
Merge branch 'master' into opencl
opencl: better choice of default configuration parameters
opencl: export pixelpipe must not be run asynchronously
Merge branch 'master' into opencl
Move static declarations into C part.