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e1ccb6632c
This adds HLSL backend support for `ImageClass::External` (ie WGSL's `external_texture` texture type). For each external texture global variable in the IR, we declare 3 `Texture2D` globals as well as a `cbuffer` for the params. The additional bindings required by these are found in the newly added `external_texture_binding_map`. Unique names for each can be obtained using `NameKey::ExternalTextureGlobalVariable`. For functions that contain ImageQuery::Size, ImageLoad, or ImageSample expressions for external textures, ensure we have generated wrapper functions for those expressions. When emitting code for the expressions themselves, simply insert a call to the wrapper function. For size queries, we return the value provided in the params struct. If that value is [0, 0] then we query the size of the plane 0 texture and return that. For load and sample, we sample the textures based on the number of planes specified in the params struct. If there is more than one plane we additionally perform YUV to RGB conversion using the provided matrix. Unfortunately HLSL does not allow structs to contain textures, meaning we are unable to wrap the 3 textures and params struct variables in a single variable that can be passed around. For our wrapper functions we therefore ensure they take the three textures and the params as consecutive arguments. Likewise, when declaring user-defined functions with external texture arguments, we expand the single external texture argument into 4 consecutive arguments. (Using NameKey::ExternalTextureFunctionArgument to ensure unique names for each.) Thankfully external textures can only be used as either global variables or function arguments. This means we only have to handle the `Expression::GlobalVariable` and `Expression::FunctionArgument` cases of `write_expr()`. Since in both cases we know the external texture can only be an argument to either a user-defined function or one of our wrapper functions, we can simply emit the names of the variables for each three textures and the params struct in a comma-separated list.