Fix positive and negative offsets causing Plug-Ins to run twice

[Etherkirby]

Closes #127

@BigRoy Suggested to call round on the plugin.order before we subtract 0.5 from it, but after some testing I found this to be a bit unsafe because of rounding errors (round(0.5) --> 0).

[BigRoy]

Not sure if it's the intended functionality - but this would reset the iterator to the order of the current plug-in. Is that intended? I'd assume that then it would process that plug-in again. No?

[Etherkirby]

Weirdly enough, at the point where publish is getting the "current pair", self.current_pair[0] is actually pointing to the next Plug-In to evaluate:

pyblish-lite/pyblish_lite/control.py

Line 91 in 6f68284

plugin = self.current_pair[0]

Here's a snippet that proves this:

from __future__ import print_function

import os

# Remove artificial delay from GUI
os.environ["PYBLISH_DELAY"] = "0"

import pyblish.api
import pyblish.lib
from pyblish_lite import control

count = {"#": 0}

class MyCollector(pyblish.api.ContextPlugin):
    order = pyblish.api.CollectorOrder

    def process(self, context):
        count["#"] += 1

class MyCollectorPosOffset(pyblish.api.ContextPlugin):
    order = pyblish.api.CollectorOrder + 0.4

    def process(self, context):
        count["#"] += 1

class MyValidatorNegOffset(pyblish.api.ContextPlugin):
    order = pyblish.api.ValidatorOrder - 0.4

    def process(self, context):
        count["#"] += 10

class MyValidator(pyblish.api.ContextPlugin):
    order = pyblish.api.ValidatorOrder

    def process(self, context):
        count["#"] += 10

pyblish.api.deregister_all_plugins()

test_plugins = (
    MyCollector, MyCollectorPosOffset, MyValidatorNegOffset, 
    MyValidator
)
for plugin in test_plugins:
    pyblish.api.register_plugin(plugin)

ctrl = control.Controller()
print("Current pair before collection:", ctrl.current_pair)
ctrl.reset()
print("Current pair after collection:", ctrl.current_pair)
assert count["#"] == 2, count

ctrl.publish()
print("Current pair after publish:", ctrl.current_pair)
# Disabling cause this fails in the current version and is getting fixed here.
# assert count["#"] == 22, count

-->

Current pair before collection: (None, None)
Current pair after collection: (<class 'pyblish.plugin.MyValidatorNegOffset'>, None)
Current pair after publish: (None, None)

TL;DR: It's actually using the order of the next Plug-In which wasn't processed yet, so no Plug-Ins are processed twice.

[mottosso]

Closing based on discussion in #127