Fix AtlasPacker ignoring semi-transparent pixels

Fixes #33106

core/image.cpp

@@ -2027,8 +2027,7 @@ Rect2 Image::get_used_rect() const {
 	for (int j = 0; j < height; j++) {
 		for (int i = 0; i < width; i++) {
 
-			bool opaque = get_pixel(i, j).a > 0.99;
-			if (!opaque)
+			if (!(get_pixel(i, j).a > 0))
 				continue;
 			if (i > maxx)
 				maxx = i;

doc/classes/Image.xml

@@ -285,7 +285,7 @@
 			<return type="Rect2">
 			</return>
 			<description>
-				Returns a [Rect2] enclosing the visible portion of the image.
+				Returns a [Rect2] enclosing the visible portion of the image, considering each pixel with a non-zero alpha channel as visible.
 			</description>
 		</method>
 		<method name="get_width" qualifiers="const">