Consolidate the Image class, fix bug (dotnet/corefx#40181)

Consolidate the Image class, fix bug (dotnet/corefx#40181)

* Add new values for Encoder, EncoderParameterValueType

* Support encoder parameters of type ValueTypePointer

* Fix an overflow error when reading EncoderParameters

* Add unit tests

* Consolidate the Image class

Share the following methods across Windows & Unix:
- FromFile
- GetEncoderParameterList
- GetPixelFormatSize
- IsAlphaPixelFormat

* Fix P/Invoke declaration

* Fix compliation on NetFX

* Skip test on NetFX

* Fix typo

* Always skip GetEncoderParameterList_ReturnsExpected on netFX

* Remove new API

* Revert changes which require an API change

* Suppress failing test

* Image: Consolidate CreateImageObject
Metafile: Consolidate initialization from native handle

* Throw SkipTestException instead of silently skipping the test

* Enable some on Unix which rely on consolidated code

* Add missing using statement

* Get SkipTestException right

Commit migrated from https://github.com/dotnet/corefx/commit/31a04af352c0046b21b2ccdf5b409203a43fae1f