Compute texture coordinates using floating point operations

Compute texture coordinates using floating point operations

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/10628
Tested-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>