Fix black pixel at (0,0)

Commit aa045d6a changed where drawCursor was called from, and changed
the previously existing drawCursor call to operate on QRect(0, 0, 0, 0)
so the following calls to draw text still used the right colors, which
may have been altered by drawCursor.

(If a block cursor is being used, and we have focus, so the whole block
is drawn (when unfocused, an outlined block is drawn), the cursor color
may conflict with the text color.  The cursor is drawn with either the
foreground color or a custom color, and text under the cursor (actually
on the text run which starts under the cursor, when "Appearance→Complex
Text Layout→Use the same attributes for whole word" is enabled) is drawn
with either the background color or another custom color.)

So, move the part of drawCursor that changes the color to use for text
under the cursor into its own method, so we don't needlessly draw a
dummy cursor at (0,0).

BUG: 462421