Closes #969 and implements the API from there, namely

static void PixelShaderEffect.RegisterEffect(UInt8[] shaderCode, Guid id, Int32 maxSamplerOffset, SamplerCoordinateMapping[] coordinateMappings, EffectBorderMode[] borderModes, CanvasImageInterpolation[] sourceInterpolations);
static PixelShaderEffect PixelShaderEffect.CreateEffect(Guid id);
static PixelShaderEffect PixelShaderEffect.RegisterAndCreateEffect(UInt8[] shaderCode, Guid id, Int32 maxSamplerOffset, SamplerCoordinateMapping[] coordinateMappings, EffectBorderMode[] borderModes, CanvasImageInterpolation[] sourceInterpolations);
static Boolean PixelShaderEffect.IsEffectRegistered(Guid id);
static void PixelShaderEffect.UnregisterEffect(Guid id);

Please look at the overall diff rather than the individual commits as I refactored it a bit.

I realize you may not want to merge this, but this is now in my forked version, so I thought I'd post it here anyway. There aren't actually that many changes. The main change to the existing code was replacing ShaderDescription as a direct member of SharedShader state with a shader_ptr instead.

The absolute time savings aren't massive but I think it's still worth it as my app may load 1000s of images as quickly as possible. I tested creating an instance of PixelShaderEffect (without realizing it) using some shader code which is not too complicated but does have a few constants. I found that on average, without the new caching code, it took 0.15ms to create each instance, whereas after registration, using the new code, it took 0.002ms to create each instance.