BUG: Fix missing Image::GetBuffer methods in CSharp

The "SWIGCSHARP" it the correct preprocessor identifier for the CSharp
swig interface.

Change-Id: Ibfb1d6d5d780ed580e07bd1bbfb65ecba8ed049d
Merge topic 'FixCSharpMissingImageGetBufferMethods' into next

b766967 BUG: Fix missing Image::GetBuffer methods in CSharp