vf_fps: use double constants for default/min/max for start_time

When using AV_NOPTS_VALUE (which expands to INT64_C(0x8000000000000000))
as union initializer for a double field, the c99 converter needs to
interpret this constant when filling the union initializer, and it is
interpreted as a positive value.

When converting AV_NOPTS_VALUE to a double, MSVC 2010 ends up with
the same positive value as the c99 converter, while MSVC 2012 gets
a negative value.

This results in an infite loop in various FATE tests on MSVC 2012.

Signed-off-by: Martin Storsjö <martin@martin.st>