be less aggressive when to disable borders
Summary:
if the control height is
2*Metrics::LineEdit_FrameWidth + option->fontMetrics.height()
it means that "it just fits", therefore still draw the frame
the qqc2-desktop style uses that logic to get default line edit
heights, so they all got disabled by just one pixel
Test Plan: tested qwidget and qml apps
Reviewers: Plasma, hpereiradacosta
Reviewed By: hpereiradacosta
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D8160