Update Corrade and Magnum with prefixed CMake options

[mosra]

Motivation and Context

Just a regular batch of (few thousand lines worth of) under-the-hood updates, plus a long-overdue renaming of CMake options to avoid clashes with other projects. Other potentially interesting changes:

• New plugin for resizing images, useful for reducing texture memory footprint or mip level generation
• New BitArray and BitArrayView containers, which are getting heavily used for data processing and scene management in the batch renderer.
• Making distinction between 3D, 2D array and cubemap images for ASTC, DDS, KTX and EXR file formats, which is especially important when dealing with mip levels (they shrink along Z for 3D images, but not for 2D arrays and cubemaps). The lack of such distinction caused considerable friction in data processing for the batch renderer, fortunately no more!

How Has This Been Tested

My CIs pass.

⚠️ To get rid of the old unprefixed CMake options and avoid unexpected clashes in the future you may want to either recreate your build directories, or delete all Corrade- and Magnum-related WITH_*, BUILD_* and TARGET_* options from your CMake cache (ccmake . in the build dir and pressing D on each such entry is a convenient way to do this).

Types of changes

• Docs change / refactoring / dependency upgrade

[dhruvbatra]

Always happy to see "a few thousand lines worth of under-the-hood updates" that make everyone's life easier :-).

[jturner65]

LGTM