Add separate initial window size

[thatcomputerguy0101]

This separates the initial window size from the max size setting, allowing a window to be created at a smaller size than its maximum. If the initial size is unspecified, it falls back on the max size, maintaining compatibility with existing configurations.

On web, the window is created from the initial size setting (falling back to max size). This may be undesired for most use cases, so a possible modification would be for the web version to ignore the initial size parameter and only use max size. However, applications can work around this by putting a platform macro on the incoming window configuration, so I felt it was likely unnecessary to create that difference in behavior.