One issue affected the situation where the provided bounds wouldn't
start at (0, 0). Such bounds are regularly acquired by calling
Rectangle::reduced().

The other issue affected the width calculation of fractional items.
The error wasn't correctly integrated during the computation, and as a
consequence the last fractional element would exhibit all the
accumulated error.

Prior to this commit all Grid calculations were carried out using
floating point numbers. The dimensions of all items would then be
rounded with the same function to calculate the integer dimensions used
for Component layout. This resulted in layout solutions where the width
or height of items with dimensions specified using the absolute Px
quantity could differ from the correctly rounded value of these values.

This commit ensures that the width and height of these items are always
correct and their cumulative error in size is distributed among items
with fractional dimensions.

Negative indices should count backwards from the last explicit
row/column number. If the resulting line number would appear before
the first explicit row/column, implicit grid lines should be added
before the first explicit row/column.