Fixes compilation errors on 1.0(?).

[dom96]

Without this I was seeing when compiling my project on 1.0.6.

C:\Users\morfe\other_projects\flippy\src\flippy.nim(912, 21) Error: type mismatch: got <int literal(0), uint8>
but expected one of:
proc `<`(x, y: int64): bool
  first type mismatch at position: 2
  required type for y: int64
  but expression 'rgba.a' is of type: uint8
proc `<`[T: SomeUnsignedInt](x, y: T): bool
  first type mismatch at position: 2
  required type for y: T: SomeUnsignedInt
  but expression 'rgba.a' is of type: uint8
proc `<`(x, y: float): bool
  first type mismatch at position: 2
  required type for y: float
  but expression 'rgba.a' is of type: uint8
proc `<`(x, y: int): bool
  first type mismatch at position: 2
  required type for y: int
  but expression 'rgba.a' is of type: uint8
proc `<`(x, y: int8): bool
  first type mismatch at position: 2
  required type for y: int8
  but expression 'rgba.a' is of type: uint8
proc `<`(x, y: int16): bool
  first type mismatch at position: 2
  required type for y: int16
  but expression 'rgba.a' is of type: uint8
proc `<`(x, y: int32): bool
  first type mismatch at position: 2
  required type for y: int32
  but expression 'rgba.a' is of type: uint8
proc `<`(x, y: float32): bool
  first type mismatch at position: 2
  required type for y: float32
  but expression 'rgba.a' is of type: uint8
proc `<`[T](x: Ordinal[T]): T
  first type mismatch at position: 2
  extra argument given
9 other mismatching symbols have been suppressed; compile with --showAllMismatches:on to see them

[treeform]

Thanks!