Fix text coming in reverse order with boxes flow disabled

[jstockwin]

Closes #398

I've got a TODO in the code to change after #396 is merged, so I'll keep this as a draft PR for now.

Example PDF

Code to reproduce:

from pdfminer.high_level import extract_text
from pdfminer.layout import LAParams

laparams = LAParams(boxes_flow=3, line_margin=0.001)
print(extract_text("test.pdf", laparams=laparams))

(Note that once #396 is merged we'll be able to do boxes_flow=None, I'm passing 3 simply to disable it)

Output before fix:

Text 4
Text 3
Long Text 1
Text 2
Text 1

Output after fix:

Text 1
Text 2
Long Text 1
Text 3
Text 4

Checklist

• I have added tests that prove my fix is effective or that my feature
  works
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md

[pietermarsman]

From the docs, section 4.2.1. Coordinate Space:

The positive x axis extends horizontally to the right and the positive y axis vertically upward, as in standard mathematical practice

Lets wait with merging this until #396 is merged.

[jstockwin]

@pietermarsman I've rebased this now that 396 is merged, so should be good to go now