Fix sign inversion bug in Base1DMapper.

There was a sign inversion bug in `Base1DMapper` that affected resizing of plots using an origin of `"top left"` and having `stretch_data = False`.  Resizing the generated plot produced a flicker, and a `RuntimeError` exception with message: "DataRange low is greater than high; unable to compute axis ticks."

This PR gives a fix.

For the record, here's a minimal script that demonstrates the bug.  Run the script below, then resize.  The plot will flicker, and you'll see the following traceback:

```
Traceback (most recent call last):
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/enable/enable/qt4/base_window.py", line 198, in paintEvent
    self.handler.paintEvent(event)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/enable/enable/qt4/base_window.py", line 50, in paintEvent
    self._enable_window._paint(event)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/enable/enable/abstract_window.py", line 467, in _paint
    self.component.draw(gc, view_bounds=(0, 0, size[0], size[1]))
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/enable/enable/component.py", line 424, in draw
    self._draw(gc, view_bounds, mode)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/enable/enable/component.py", line 776, in _draw
    self._dispatch_draw(layer, gc, view_bounds, mode)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/enable/enable/container.py", line 257, in _dispatch_draw
    my_handler(gc, view_bounds, mode)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/enable/enable/container.py", line 297, in _draw_container_underlay
    self._draw_underlay(gc, view_bounds, mode)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/enable/enable/component.py", line 884, in _draw_underlay
    underlay.overlay(self, gc, view_bounds, mode)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/chaco/chaco/axis.py", line 222, in overlay
    self._draw_component(gc, view_bounds, mode, component)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/chaco/chaco/axis.py", line 247, in _draw_component
    self._compute_tick_positions(gc, component)
  File "/Users/mdickinson/Enthought/Source/meta-geophysics/meta-ETS/chaco/chaco/axis.py", line 463, in _compute_tick_positions
    raise RuntimeError, "DataRange low is greater than high; unable to compute axis ticks."
RuntimeError: DataRange low is greater than high; unable to compute axis ticks.
```

Here's the script.

```python
from numpy import linspace
from scipy.special import jn

from chaco.api import ArrayPlotData, Plot
from enable.api import Component, ComponentEditor
from traits.api import HasTraits, Instance
from traitsui.api import Group, Item, View


class Demo(HasTraits):
    plot = Instance(Component)

    def _plot_default(self):
        x = linspace(-2.0, 10.0, 100)
        plot_data = ArrayPlotData(x=x, y=jn(3, x))
        plot = Plot(plot_data, origin="top left")
        plot.value_mapper.stretch_data = False
        plot.plot(("x", "y"))
        return plot

    traits_view = View(
        Group(
            Item(
                'plot',
                editor=ComponentEditor(),
                show_label=False,
            ),
        ),
        resizable=True,
    )


if __name__ == "__main__":
    demo = Demo()
    demo.configure_traits()