glClearBuffer implementation

Implemented glClearBuffer* functions.
Right now, all colors are represented
as floats internally, but should that
change, and integer types be supported,
we'd be able to remove the casts to
float here.

Change-Id: Ie785858d77ce7ac03b78fca9e43ac922c15e76e0
Reviewed-on: https://swiftshader-review.googlesource.com/3004
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>