VideoCommon: add additional texture sampler types to ShaderAsset

[iwubcode]

This adds a couple of additional sampler definitions to the ShaderAsset. This allows for distinguishing between a 2d sampler defined by assets and a 2d sampler that was defined by an alternate source (ex: Dolphin has a definition for an array of samplers in PixelShaderGen for instance). This distinction isn't super necessary at the moment but will be important later on if a user wants to add 2d textures with their own sampler information and independent resolution (needs #12136 )

I add two separate entries. One called Type_SamplerArrayShared_Main that denotes the main texture, in a sampler array this is where the sampler would come from and also sets the requirement for the size of the texture. Type_SamplerArrayShared_Additional is an additional texture which follows the sampler rules defined by Type_SamplerArrayShared_Main (which isn't a new concept, that's how texture sampler arrays work!)