GL was missing a check whether given format is available on a target
(for example double types are not on ES), and for Vulkan we need something
similar to pixel format mapping as well.
Better for checking accidents, as picking a wrong primitive / index type
can lead to *serious* rendering issues. Similarly to a change done to
(Compressed)PixelFormat in 2019.10.
The stripped-down versions didn't actually test for correct order
because there was no external API result to check against expected
output. That's fixed now, plus clarified some comments and removed
copypasted nonsense.
GL has an extension, but only for ES, not on desktop. Vulkan has
nothing yet (due to there being just ARM that implements it, no other
vendor), except those being listed in a KTX format specification.
Deprecated for 2018.04, it's been almost a year since. Whoever is using
Magnum regularly updated already, and who not can always upgrade
gradually (2018.02, 2018.04, 2018.10, 2019.01 etc.).