GL_R11F_G11F_B10F is a renderable format (in the EXT_color_buffer_float
extension) with unsigned floating-point values, which we implement using
(signed) half- or single-precision floating-point types. So we need to
clamp values to a positive range before writing to them.

Change-Id: Ic21a5b0b33905c0aeab35299fc268158f8c679f9
Reviewed-on: https://swiftshader-review.googlesource.com/15448Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>