Bevy UI was using the MeasureFunc that preserves aspect ratio for both images and text.
This meant that the cross-axis of flex items containing text would be calculated incorrectly sometimes
depending on how the aspect ratio of the text compared to the size of the containing node.

changes:
    * Added parameter preserve_aspect_ratio to the upsert_leaf function.
    * The MeasureFunc only preserves the aspect ratio when preserve_aspect_ratio is true.
    * flex_node_system queries for UiImage before it calls upsert_leaf
    and sets preserve_aspect_ratio appropriately.

…aspect_ratio field to CalculatedSize which is

set to true for image nodes by update_image_calculated_size_system.