software_isp: debayer_cpu: Add multi-threading support

Add CPU soft ISP multi-threading support.

Benchmark results for the Arduino Uno-Q with a weak CPU which is good for
performance testing, all numbers with an IMX219 running at
3280x2464 -> 3272x2464:

1 thread : 147ms / frame, ~6.5 fps
2 threads:  80ms / frame, ~12.5 fps
3 threads:  65ms / frame, ~15 fps

Adding a 4th thread does not improve performance.

Tested-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com> # ThinkPad X1 Yoga Gen 7 + ov2740
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Signed-off-by: Hans de Goede <johannes.goede@oss.qualcomm.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Documentation/runtime_configuration.rst

@@ -51,6 +51,7 @@ file structure:
       measure:
         skip: # non-negative integer, frames to skip initially
         number: # non-negative integer, frames to measure
+      threads: # integer >= 1, number of render threads to use, default 2
 
 Configuration file example
 --------------------------
@@ -84,6 +85,7 @@ Configuration file example
        measure:
          skip: 50
          number: 30
+       threads: 2
 
 List of variables and configuration options
 -------------------------------------------
@@ -167,6 +169,12 @@ software_isp.measure.skip, software_isp.measure.number
 
    Example `number` value: ``30``
 
+software_isp.threads
+   Number of render threads the software ISP uses when using the CPU.
+   This must be between 1 and 8 and the default is 2.
+
+   Example value: ``2``
+
 Further details
 ---------------