Let the fused shader sample a window of a larger source

A photograph larger than one texture has to be developed from a part of
it or from a reduced copy of it. The generated prologue measured the frame
by the bound texture, so either would have moved every crop, warp and
grain seed. Two uniform vec4s now say which part of the frame the texture
holds and how large the frame is; the sampler maps into the texture
through to_window, and a texture holding the whole frame samples exactly
as before ((uv - 0) / 1 is uv to the bit).

core/dr-pipeline/src/framing.rs

@@ -1298,7 +1298,8 @@ impl Framing {
         // count active stages, and a neutral graph must generate none.
         if !self.is_active() {
             return "    // Source position, normalised and centred: the whole frame, unrotated.
-    let src_dims = textureDimensions(source);
+    let tex_dims = textureDimensions(source);
+    let src_dims = select(tex_dims, vec2<u32>(u.source_full.xy), u.source_full.x > 0.5);
     let aspect = vec2<f32>(f32(src_dims.x) / f32(src_dims.y), 1.0);
     let uv = (vec2<f32>(gid.xy) + vec2<f32>(0.5)) / vec2<f32>(dims);
     var p = (uv - vec2<f32>(0.5)) * aspect;
@@ -1314,7 +1315,8 @@ impl Framing {
     // warp chain expects: the centre is (0, 0) and the radius is 1 at the
     // corner. Working here rather than in pixels is what makes the map
     // independent of the resolution being rendered at.
-    let src_dims = textureDimensions(source);
+    let tex_dims = textureDimensions(source);
+    let src_dims = select(tex_dims, vec2<u32>(u.source_full.xy), u.source_full.x > 0.5);
     let aspect = vec2<f32>(f32(src_dims.x) / f32(src_dims.y), 1.0);
     var uv = (vec2<f32>(gid.xy) + vec2<f32>(0.5)) / vec2<f32>(dims);
 ",