libcamera: Rename "shutter speed" to "exposure time"

The terms "shutter" and "shutter speed" are used through libcamera to
mean "exposure time". This is confusing, both due to "speed" being used
as "time" while it should be the inverse (i.e. a maximum speed should
correspond to the minimum time), and due to "shutter speed" and
"exposure time" being used in different places with the same meaning.

To improve clarity of the code base and the documentation, use "exposure
time" consistently to replace "shutter speed".

This rename highlighted another vocabulary issue in libcamera. The
ExposureModeHelper::splitExposure() function used to document that it
splits "exposure time into shutter time and gain". It has been reworded
to "split exposure into exposure time and gain". That is not entirely
satisfactory, as "exposure" has a defined meaning in photography (see
https://en.wikipedia.org/wiki/Exposure_(photography)) that is not
expressed as a duration. This issue if left to be addressed separately.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Acked-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/control_ids_core.yaml

@@ -102,7 +102,7 @@ controls:
         Specify an exposure mode for the AE algorithm to use.
 
         The exposure modes specify how the desired total exposure is divided
-        between the shutter time and the sensor's analogue gain. They are
+        between the exposure time and the sensor's analogue gain. They are
         platform specific, and not all exposure modes may be supported.
       enum:
         - name: ExposureNormal
@@ -135,8 +135,7 @@ controls:
   - ExposureTime:
       type: int32_t
       description: |
-        Exposure time (shutter speed) for the frame applied in the sensor
-        device.
+        Exposure time for the frame applied in the sensor device.
 
         This value is specified in micro-seconds.
 
@@ -463,14 +462,13 @@ controls:
         values to be the same. Setting both values to 0 reverts to using the
         camera defaults.
 
-        The maximum frame duration provides the absolute limit to the shutter
-        speed computed by the AE algorithm and it overrides any exposure mode
+        The maximum frame duration provides the absolute limit to the exposure
+        time computed by the AE algorithm and it overrides any exposure mode
         setting specified with controls::AeExposureMode. Similarly, when a
         manual exposure time is set through controls::ExposureTime, it also
         gets clipped to the limits set by this control. When reported in
-        metadata, the control expresses the minimum and maximum frame
-        durations used after being clipped to the sensor provided frame
-        duration limits.
+        metadata, the control expresses the minimum and maximum frame durations
+        used after being clipped to the sensor provided frame duration limits.
 
         \sa AeExposureMode
         \sa ExposureTime