Calc: The + and - operators must always be surrounded by whitespace

[yoavf]

Currently, rtlcss will convert background-position: calc(100% - 10px) center; to background-position: calc(100%-(100% - 10px)) center; which is mostly correct, except for the missing whitespace around the added -.

With Chrome 52, this cause misalignment. Manually adding the whitespace fixes the visual problem.

From mdn:

The + and - operators must always be surrounded by whitespace. The operand of calc(50% -8px) for instance will be parsed as a percentage followed by a negative length, an invalid expression, while the operand of calc(50% - 8px) is a percentage followed by a minus sign and a length. Even further, calc(8px + -50%) is treated as a length followed by a plus sign and a negative percentage.
The * and / operators do not require whitespace, but adding it for consistency is allowed, and recommended.