Optimize PixelFormat creation and PixelFormatConverter conversion speed (#9861)

* Simplify CanConvertFrom

* Simplify CanConvertTo

* Simplify ConvertFromString

* Simplify ConvertFrom

* Use pattern-matching in ConvertTo

* Improve performance of internal PixelFormat ctor

* Shave off 200 lines, improve readibility by using switch expressions

* Use stackallocated buffer in GetPixelFormatFlagsFromGuid

* Use stackallocated buffer in GetPixelFormat

* Mark members of PixelFormat that could be readonly as readonly

* Add documentation, remove redundant usings on PixelFormatConverter

* Speed up Guid constructor, make Guid readonly, remove redundant usings

* Mark members we made readonly in ref projects as well

* Stop allocating a wrapper over Masks array we allocate exclusively for the caller

* Fix post-merge issues

* Swap to readonly collection based on #9860 discussion

* Remove temp variable

Author: h3xds1nz