Add crossfade_or_crop_next resize shader example

wiki/examples/resize_custom_shader.frag

@@ -159,6 +159,50 @@ vec4 stretch_or_crop_next(vec3 coords_curr_geo, vec3 size_curr_geo) {
     return color;
 }
 
+// Example: cropped next texture if it's bigger than the current geometry, and
+// crossfade between previous and next texture otherwise.
+vec4 crossfade_or_crop_next(vec3 coords_curr_geo, vec3 size_curr_geo) {
+    vec3 coords_next_geo = niri_curr_geo_to_next_geo * coords_curr_geo;
+    vec3 coords_prev_geo = niri_curr_geo_to_prev_geo * coords_curr_geo;
+
+    vec3 coords_crop = niri_geo_to_tex_next * coords_next_geo;
+    vec3 coords_stretch = niri_geo_to_tex_next * coords_curr_geo;
+    vec3 coords_stretch_prev = niri_geo_to_tex_prev * coords_curr_geo;
+
+    // We can crop if the current window size is smaller than the next window
+    // size. One way to tell is by comparing to 1.0 the X and Y scaling
+    // coefficients in the current-to-next transformation matrix.
+    bool can_crop_by_x = niri_curr_geo_to_next_geo[0][0] <= 1.0;
+    bool can_crop_by_y = niri_curr_geo_to_next_geo[1][1] <= 1.0;
+    bool crop = can_crop_by_x && can_crop_by_y;
+
+    vec4 color;
+
+    if (crop) {
+        // However, when we crop, we also want to crop out anything outside the
+        // current geometry. This is because the area of the shader is unspecified
+        // and usually bigger than the current geometry, so if we don't fill pixels
+        // outside with transparency, the texture will leak out.
+        //
+        // When crossfading, this is not an issue because the area outside will
+        // correspond to client-side decoration shadows, which are already supposed
+        // to be outside.
+        if (coords_curr_geo.x < 0.0 || 1.0 < coords_curr_geo.x ||
+                coords_curr_geo.y < 0.0 || 1.0 < coords_curr_geo.y) {
+            color = vec4(0.0);
+        } else {
+            color = texture2D(niri_tex_next, coords_crop.st);
+        }
+    } else {
+        // If we can't crop, then crossfade.
+        color = texture2D(niri_tex_next, coords_stretch.st);
+        vec4 color_prev = texture2D(niri_tex_prev, coords_stretch_prev.st);
+        color = mix(color_prev, color, niri_clamped_progress);
+    }
+
+    return color;
+}
+
 // This is the function that you must define.
 vec4 resize_color(vec3 coords_curr_geo, vec3 size_curr_geo) {
     // You can pick one of the example functions or write your own.