Include what you use with signbit.

Include what you use with signbit.

Include cmath in a few source files which use signbit and a relying on
magic to happen to use it.

Also Fix nuttiness in SampleClip. No need to #define single character
identifiers.

Change-Id: Iae3352d0cab9aaa6c37d6424f064b3d86fa2e011
Reviewed-on: https://skia-review.googlesource.com/4626
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>