mtlb_int8 Matlab int8 emulation function

## Description

Matlab and Scilab `int8` behave differently :

- Beyong the main interval bounds, Scilab's `int8()` wraps, while Matlab's `mtlb_int8()` saturates.
- For non integer values, Scilab's `int8()` truncates the fractional part, while Matlab's `mtlb_int8()` rounds to the nearest integer.

x | int8(x) | mtlb_int8(x)
--- | --- | ---
%nan | 0 | 0
-%inf | -128 | -128
-130 | 126 | -128
-129 | 127 | -128
-128 | -128 | -128
-127 | -127 | -127
-80.7 | -80 | -81
-80.5 | -80 | -81
-80.4 | -80 | -80
10.2 | 10 | 10
10.5 | 10 | 11
10.51 | 10 | 11
127 | 127 | 127
128 | -128 | 127
129 | -127 | 127
%inf | 127 | 127

`mtlb_int8` has not to be used for hand coded functions.

The function `mtlb_int8(A)` is used by `mfile2sci` to replace `int8(A)` when it is not possible to know what is the input while porting Matlab code to Scilab. This function then determines the correct semantic at run time.