* Remove Tizen bitmap code; simplify platform detection

Remove platform-specific Tizen bitmap implementation and related build define, and simplify platform mode detection logic. Deleted Tizen bitmap files (BitmapMixins.cs, PlatformBitmapLoader.cs, TizenBitmap.cs) and removed the TIZEN DefineConstants entry from Directory.Build.targets. Updated DefaultPlatformModeDetector to drop NETFX_CORE guards and to remove Tizen from conditional compilation branches, unifying how entry assembly/base directory and design-environment checks are performed.

* Update Splat.Drawing.csproj