LibGfx: Redirect PNG errors and warnings to our own logging functions

[rgret-dev]

Before, libpng would use its own internal logging mechanism to print non-fatal errors and warnings to stdout/stderr. This made it confusing when trying to search the Ladybird codebase for those messages as they didn't exist.

This commit uses png_set_error_fn from libpng to redirect those messages to our own custom logging functions instead.

cc: @LucasChollet