TextInput: Fix cursor height

On many platforms, the current cursor looks too tall. The logic used
in this patch is inspired by what we have in QTextLayout::drawCursor().
It still looks too tall on Mac, but so it does in Qt Widgets.

QQuickTextInput::positionToRectangle() has also been updated for consistency.

Change-Id: I69b8ad246238d54db370db639a319a3edba6d78a
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>