Modified the texture coordinate computation to use floating
point operations when sampling floating point textures to
increase accuracy and remove useless type conversions.

Bug swiftshader:76

Change-Id: I76e5fd9a68aa6ba5eb21524edb8c6c6183e5430e
Reviewed-on: https://swiftshader-review.googlesource.com/10628Tested-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>