@BlackPhlox kindly pointed out and resolved a couple of inconsistencies in the bevy logo:
* The arc of the first bird's back had three vertices right next to each other, which created a noticeable sharp edge. This replaces them with a single vertex.
* The bottom part of the tail had a sharp edge, which was inconsistent with the top part of the tail. This was rounded out to mirror the top part.
I also took the chance to clean up some of the variants and (hopefully) improve the "bevy_logo_light_dark_and_dimmed" variant to improve how it renders on dark themes.
