Prevent weird initial window dimensions caused by subpixel computations

As discussed in #41573 (closed), this problem is worked around by letterboxing, i.e. content cannot observe these weird off-by-one-pixel window sizes because the browser element gets correctly rounded with subpixel computation.

Since we want to protect to some extent also users who disable letterboxing, here we're implementing for them a similar mechanism to set the default rounded window size, except it shouldn't cause any visible margin / window estate reduction.