Update cameras.xml with values for Canon EOS 550D

[yattaro]

The white point value for the Canon EOS 550D/T2i/Kiss X4 when the image is below 200 ISO is missing, resulting in blown highlights. A sample raw image shot on my 550D where this issue can be seen is here.

This PR sets the white point for this camera to 13583 when below 200 ISO.

[LebedevRI]

As mentioned in darktable-org/darktable#15636 (comment), we really should read these from EXIF.

Could you please take a shot on every ISO level, convert them to DNG and process them with dngmeta.rb to obtain the full set of values?

[yattaro]

As mentioned in darktable-org/darktable#15636 (comment), we really should read these from EXIF.

Could you please take a shot on every ISO level, convert them to DNG and process them with dngmeta.rb to obtain the full set of values?

Got it, here's the resulting output of dngmeta.rb:

invsensors {[2048, 13926]=>[100], [2054, 15000]=>[200, 400, 800, 1600, 3200, 6400, 12800]}

sensors [[100, [2048, 13926]], [200, [2054, 15000]], [400, [2048, 15000]], [800, [2048, 15000]], [1600, [2048, 15000]], [3200, [2048, 15000]], [6400, [2054, 15000]], [12800, [2054, 15000]]]
mostfrequent [[2054, 15000], [200, 400, 800, 1600, 3200, 6400, 12800]]
invsensors {[2048, 13926]=>[100]}

        <Camera make="Canon" model="Canon EOS REBEL T2i">
                <ID make="" model="">Canon EOS 550D</ID>
                <Sensor black="2054" white="15000"/>
                <Sensor black="2048" white="13926" iso_list="100"/>
        </Camera>

I've updated this PR accordingly.

[LebedevRI]

@yattaro thank you!