photo1.pl: Prevent blank window when opened

[chrstphrchvz]

• Simply calling after(2000) blocks events for 2 seconds, and prevents demo from appearing. Instead, rewrite after() with a subroutine to update transparency.
• Remove unneeded idletasks().

As done in Tcl::pTk: https://github.com/chrstphrchvz/perl-tcl-ptk/commit/3177cb6