This case occurs when the cursor is at the start of the table, the
table is at the start of the text, and there is no selection.

This fixes inconsistencies shown in the tests I added for the
WM_GETDLGCODE.  The tests covered different cases handled by the
current implementation in order to show that the native implementation
is simpler for all these cases.

This makes more sense than the other way around.

This allows us to skip BGRA->RGBA color conversion for vertex attributes if
this extension is present.