Fix cell overflow button overlapping scroll bar and not truncating text with row header

[mattrunyon]

This fixes 2 small issues with the cell overflow button.

1. Cell overflow button was overlapping the horizontal scroll bar. Now it draws under the scroll bar. It should still show even if the scroll bar partially covers a row because you can have the row covered by 1-2 pixels and you will see all content in the cell still. The overflow not appearing because the border is overlapped by the scroll bar could be frustrating to a user.
2. Cells were not truncating properly when IrisGridTheme.rowHeaderWidth was non-zero. The header width was not accounted for

[codecov]

Codecov Report

Merging #544 (967bf61) into main (ac30f27) will increase coverage by 0.00%.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##             main     #544   +/-   ##
=======================================
  Coverage   36.38%   36.39%           
=======================================
  Files         390      390           
  Lines       28503    28505    +2     
  Branches     6731     6731           
=======================================
+ Hits        10371    10373    +2     
  Misses      17786    17786           
  Partials      346      346           

Flag	Coverage Δ
unit	36.39% <75.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/iris-grid/src/IrisGridRenderer.js	24.31% <75.00%> (+0.34%)	⬆️

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