Fix rounding when sizing 1x1 textures

Fix rounding when sizing 1x1 textures

When 1x1 textures were sized to the actual square size, this could lead
to bitmaps that were 1 pixel too small, presumably because of rounding.
We now add 0.99 pixel to the desired size of the texture image before
calculating the scaling factor (in floating arithmetic) that has to be
passed to the cairo scaling routine, and this seems to solve the problem.