plugins: input-raw: check device properties only when filedesc changes
plugins: input-raw: check device properties only when filedesc changes

remember whether the device sets pressure values. If it doesn't, we must do
it ourselves. But only when the file descriptor changes, we have to do
device property checks again.

this should improve performance by saving time during ts_read().

kergoth/tslib,kergoth/tslib,kergoth/tslib,kergoth/tslib