use stub PNG encoder for Windows DLL builds

Windows builds a shared library (DLL) which requires all symbols to be
resolved at link time. The serialization code references SkPngEncoder::Encode,
but we don't actually need PNG encoding for PathOps.

Use skia_use_no_png_encode=true to provide a stub implementation that
doesn't require libpng or zlib dependencies.

Fixes the linker error:
error LNK2019: unresolved external symbol SkPngEncoder::Encode

Author: anthrotype

build_skia.py

@@ -45,12 +45,12 @@
 
 if sys.platform == "win32":
     # On Windows, we build a shared library (DLL) which requires all symbols to be
-    # resolved at link time. PNG encoder is referenced by serialization code, so we
-    # must enable PNG encoding (and zlib which PNG depends on) for Windows DLL builds.
+    # resolved at link time. PNG encoder is referenced by serialization code, but we
+    # don't actually need PNG encoding for PathOps. Use the stub implementation instead.
     SKIA_BUILD_ARGS.extend([
-        "skia_use_libpng_encode=true",
-        "skia_use_libpng_decode=true",
-        "skia_use_zlib=true",
+        "skia_use_no_png_encode=true",
+        "skia_use_libpng_decode=false",
+        "skia_use_zlib=false",
     ])
 else:
     SKIA_BUILD_ARGS.extend([