UI: Support fractional scaling for Canvas/Base size

Without this change, new profiles created on systems using a fractional
scaling factor (e.g., 125%, 150%, 175%) will get an incorrect (X, Y)
pair for the new canvas size. For a display with 125% scaling, OBS would
set (1536, 864) instead of (1920, 1080). With this change, it will set
(1920, 1080).
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