libpng: Do not show warnings in the browser console (#19633)

Such warnings can include things like:

libpng warning: iCCP: known incorrect sRGB profile

Author: HCLJason

tools/ports/libpng.py

@@ -341,7 +341,7 @@ def show():
 #define PNG_USER_MEM_SUPPORTED
 #define PNG_USER_TRANSFORM_INFO_SUPPORTED
 #define PNG_USER_TRANSFORM_PTR_SUPPORTED
-#define PNG_WARNINGS_SUPPORTED
+#undef PNG_WARNINGS_SUPPORTED
 #define PNG_WRITE_16BIT_SUPPORTED
 #define PNG_WRITE_ANCILLARY_CHUNKS_SUPPORTED
 #define PNG_WRITE_BGR_SUPPORTED