Add breakpoint flag to convert_url()

[pereirinha]

This will allow applying the default transformations when calling the_post_thumbnail().

[dugajean]

The solution looks clean to me, but could you explain how this fixes the issue at hand?

[DavidCramer]

@dugajean - the the_post_thumbnail() calls the image_downsize filter which allows us to replace a sized URL for an image. The problem with that is that we filter this by converting the URL, which is an exact copy without global transformations, since convert_url was initially made to convert breakpoints by passing an array of transformations from the parent.
The result is a clean, untransformed URL.

The flag that @pereirinha added, lets us selectively tell it to apply globals transformations, with it set to false in the breakpoint context.

This however is only a partial fix, since it doesn't take into account if the featured image has it's own transformations. I have pushed a fix that uses the correct way, by converting the primary URL, and having the breakpoints part be an accessory rather than an override.
I also renamed the $breakpoint flag to better explain it's purpose; Overwrite Transformations

[dugajean]

Got it @DavidCramer, thanks for explaining. 😄