feature #7756 [Validator] Add min/max amount of pixels to Image constraint (akeeman)

This PR was submitted for the master branch but it was squashed and merged into the 3.4 branch instead (closes #7756).

Discussion
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[Validator] Add min/max amount of pixels to Image constraint

Documentation for symfony/symfony#22288

Commits
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32f90ee Add minPixels and maxPixels constraints

Author: wouterj

reference/constraints/Image.rst

@@ -274,6 +274,22 @@ maxHeight
 If set, the height of the image file must be less than or equal to this
 value in pixels.
 
+minPixels
+~~~~~~~~~
+
+**type**: ``integer``
+
+If set, the amount of pixels of the image file must be greater than or equal to this
+value.
+
+maxPixels
+~~~~~~~~~
+
+**type**: ``integer``
+
+If set, the amount of pixels of the image file must be less than or equal to this
+value.
+
 maxRatio
 ~~~~~~~~
 
@@ -363,6 +379,22 @@ Minimum height expected is {{ min_height }}px.``
 
 The error message if the height of the image is less than `minHeight`_.
 
+maxPixelsMessage
+~~~~~~~~~~~~~~~~
+
+**type**: ``string`` **default**: ``The image has to many pixels ({{ pixels }} pixels).
+Maximum amount expected is {{ max_pixels }} pixels.``
+
+The error message if the amount of pixels of the image exceeds `maxPixels`_.
+
+minPixelsMessage
+~~~~~~~~~~~~~~~~
+
+**type**: ``string`` **default**: ``The image has to few pixels ({{ pixels }} pixels).
+Minimum amount expected is {{ min_pixels }} pixels.``
+
+The error message if the amount of pixels of the image is less than `minPixels`_.
+
 maxRatioMessage
 ~~~~~~~~~~~~~~~