disable legacy PNG encoding in image serialization

Add SK_DISABLE_LEGACY_PNG_WRITEBUFFER preprocessor define to remove
PNG encoder dependency from SkWriteBuffer serialization code. This
allows Windows DLL builds to succeed without requiring PNG encoder
implementation, as DLLs must resolve all symbols at link time.

This matches Chromium's approach (see chromium/src@c97e564) and
aligns with Skia's Bazel build which already disables this by default.

Author: anthrotype

build_skia.py

@@ -44,6 +44,7 @@
     "skia_enable_spirv_validation=false",
     "skia_use_lua=false",
     "skia_use_wuffs=false",
+    'extra_cflags=["-DSK_DISABLE_LEGACY_PNG_WRITEBUFFER"]',
 ]
 if sys.platform != "win32":
     # On Linux, I need this flag otherwise I get undefined symbol upon importing;