When reading an inexistent I/O port in the Sinclair Spectrum,
the value read not always is 255. That only happens when the
ULA is painting the BORDER. When painting the PAPER, the bus
presents the current value being read by the ULA from the
video memory.

This effect is used by some games to synchronize with the beam.
They can use a naive way, by just waiting for a non-255 value
to appear in the bus, thus knowing that now the beam has
started painting the PAPER, or checking that value and waiting
for an specific one (usually a color attribute value), thus
being able to detect when an specific line is being painted.

This patch emulates this behavior, thus allowing these games
to run in the emulator.

Fixes gasman#14