image_get_intermediate_size() sometimes skips image sizes

[flixos90]

Sometimes it can happen that an image size is not considered in image_get_intermediate_size() although it applies for the array.

This is because the product of the image width and height is used as array key, however there are cases where different image sizes could share the same product (for example 150x300 and 200x225).

[flixos90]

Fixes the bug by allowing multiple sizes of the same width*height

[joemcgill]

Yup, this is indeed a bug. Will need unit tests, but am moving to 4.6 for consideration.

[joemcgill]

If we end up going with something like 34384.2.diff to fix #34384, we should end up avoiding this issue.

[joemcgill]

In 38086:

Media: Prevent image_get_intermediate_size() from returning cropped images.

When $size is passed to image_get_intermediate_size() as an array of width
and height values and an exact image size matching those values isn't available,
the function loops through the available attachment sizes and returns the
smallest image larger than the requested dimensions with the same aspect ratio.

The aspect ratio check is skipped for the 'thumbnail' size to provide a fallback
for small sizes when no other image option is available. This resulted in a poor
selection when the size requested was smaller than the 'thumbnail' dimensions
but a larger size matching the requested ratio existed.

This refactors the internals of image_get_intermediate_size() to ensure the
'thumbnail' size is only returned as a fallback to small sizes once all other
options have been considered, and makes the control flow easier to follow.

This also introduces a new helper function, wp_image_matches_ratio() for
testing whether the aspect ratios of two sets of dimensions match. This function
is also now used in wp_calculate_image_srcset() during the selection process.

Props flixos, joemcgill.
Fixes #34384, #34980.