Develop a linear DNG from windows and reduced copies of it

DemosaicedImage::linear_rgb16_window uploads part of a linear DNG, or a
box-reduced copy of it, and says where it sits in the frame; size() now
reports the frame and texture_size() the texels, and the fused pass
writes the window into the shader's uniforms. EditGraph::source_region
finds the part of the source a view reads, and tiles::plan cuts a render
too large for one texture into halo-grown, grid-aligned tiles.

The GPU test renders frames a tile at a time from their own windows and
compares them with the whole: identical for point operations, within one
code value when straightened with clarity on.

core/dr-gpu/src/mask.rs

@@ -928,7 +928,7 @@ impl MaskPass {
         // the only readers and they are skipped in that case.
         let source_step = match source {
             Some(image) => {
-                let (sw, sh) = image.size();
+                let (sw, sh) = image.texture_size();
                 [
                     sw as f32 / width.max(1) as f32,
                     sh as f32 / height.max(1) as f32,