Raster images: transparency off draws pixels opaque; TRANSPARENCY command

An image's transparency flag decides whether transparent pixels show what
is behind the image or draw in their stored colour: the image pass and PDF
plots now follow it (it was ignored, so every PNG alpha showed through).
TRANSPARENCY selects images and asks for the mode, defaulting to the first
image's current one.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/shaders/image.wgsl

@@ -28,7 +28,7 @@ struct Uniforms {
 struct ImageParams {
     opacity:    f32,
     draw_depth: f32,   // signed (-1,1) draw-order bias; 0 = neutral
-    _pad1:      f32,
+    use_alpha:  f32,   // 1 = pixel alpha applies, 0 = pixels draw opaque
     _pad2:      f32,
 };
 @group(1) @binding(2) var<uniform> img_params: ImageParams;
@@ -63,5 +63,6 @@ fn vs_main(in: VertIn) -> VertOut {
 @fragment
 fn fs_main(in: VertOut) -> @location(0) vec4<f32> {
     let col = textureSample(img_texture, img_sampler, in.uv);
-    return vec4<f32>(col.rgb, col.a * img_params.opacity);
+    let alpha = select(1.0, col.a, img_params.use_alpha > 0.5);
+    return vec4<f32>(col.rgb, alpha * img_params.opacity);
 }