[rust png] Rename `image_diff_png.cpp` to `image_diff_png_libpng.cpp`.

This makes room for introducing `image_diff_png_skia.cpp` in a future
CL (see https://pdfium-review.googlesource.com/c/pdfium/+/135630).
This is a separate CL to limit the usage of `Bypass-Check-License`
to just this small CL.

Bug: 444045690
Change-Id: I03ca3068fc7dea9721dea8c02d035b7367b41f60
Bypass-Check-License: This CL just renames a file.
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/136271
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@google.com>

testing/image_diff/BUILD.gn

diff --git a/testing/image_diff/BUILD.gn b/testing/image_diff/BUILD.gn
index a311ec9..a0e013c 100644
--- a/testing/image_diff/BUILD.gn
+++ b/testing/image_diff/BUILD.gn
@@ -7,7 +7,7 @@
 source_set("image_diff") {
   testonly = true
   sources = [
-    "image_diff_png.cpp",
+    "image_diff_png_libpng.cpp",
     "image_diff_png.h",
   ]
   configs += [

testing/image_diff/image_diff_png_libpng.cpp

diff --git a/testing/image_diff/image_diff_png.cpp b/testing/image_diff/image_diff_png_libpng.cpp
similarity index 100%
rename from testing/image_diff/image_diff_png.cpp
rename to testing/image_diff/image_diff_png_libpng.cpp