libcamera: formats: Add fields to info to ease calculating stride

Packed formats make it difficult to calculate stride as well as
frame size with the fields that PixelFormatInfo currently has.
bitsPerPixel is defined as the average number of bits per pixel, and
only counts effective bits, so it is not useful for calculating
stride and frame size.

To fix this, we introduce a concept of a "pixel group". The size of this
group is defined as the minimum number of pixels (including padding)
necessary in a row when the image has only one column of effective
pixels. The pixel group has one more attribute, that is the "bytes per
group". This determines how many bytes one pixel group consumes. These
are the fields pixelsPerGroup and bytesPerGroup that are defined in this
patch. Defining these two values makes it really simple to calculate
bytes-per-line, as ceil(width / pixelsPerGroup) * bytesPerGroup, where
width is measured in number of pixels. The ceiling accounts for padding.

The pixel group has another contraint, which is that the pixel group
(bytesPerGroup and pixelsPerGroup) is the smallest repeatable unit. What
this means is that, for example, in the IPU3 formats, if there is only
one column of effective pixels, it looks like it could be fit in 5 bytes
with 3 padding pixels (for a total of 4 pixels over 5 bytes). However,
this unit is not repeatable, as at the 7th group in the same row, the
pattern is broken. Therefore, the pixel group for IPU3 formats must be
25 pixels over 32 bytes.

Clearly, pixelsPerGroup must be constant for all planes in the format.
The bytesPerGroup then, must be a per-plane attribute. There is one more
field, verticalSubSampling, that is per-plane. This is simply a divider,
to divide the number of rows of pixels by the sub-sampling value, to
obtain the number of rows of pixels for the subsampled plane.

For example, for something simple like BGR888, it is self-explanatory:
the pixel group size is 1, and the bytes necessary is 3, and there is
only one plane with no (= 1) vertical subsampling. For YUYV, the
CbCr pair is shared between two pixels, so even if you have only one
pixel, you would still need a padded second Y, therefore the pixel
group size is 2, and bytes necessary is 4 (as opposed to 1 and 2). YUYV
also has no vertical subsampling. NV12 has a pixel group size of 2
pixels, due to the CbCr plane. The bytes per group then, for both
planes, is 2. The first plane has no vertical subsampling, but the
second plane is subsampled by a factor of 2.
The IPU3 formats are also self-explanatory, as they are single-planar,
and have a pixel group size of 25, consuming 32 bytes. Although a
comment in the driver suggests that it should be 50 and 64,
respectively, this is an attribute of the driver, and not the format, so
this shall be set by the ipu3 pipeline handler.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

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