Use fixed width integers for CSS length type

[rodarima]

The current type for a CSS "length" like 80px or 20% is represented internally as a single int type, which doesn't have a fixed length. In C and C++, there is only a guarantee that an int will hold at least 16 bits, but the current assumption is that it will hold at least 32 bits.

| <------   integer value   ------> |

+---+ - - - +---+---+- - - - - -+---+---+---+---+
|          integer part             |   type    |
+---+ - - - +---+---+- - - - - -+---+---+---+---+
| integer part  | decimal fraction  |   type    |
+---+ - - - +---+---+- - - - - -+---+---+---+---+
 n-1          15  14              3   2  1   0

| <------ fixed point value ------> |

Furthermore, as we introduce more length types we are going to run out of length bits for the value itself. A simple option could be to define the length as something like this:

typedef struct CssLength {
    CssLengthType type;
    union {
        int i;
        float f;
    };
} CssLength;

Which will duplicate the size required to store lengths to 64 bits, as the type will likely use 32 bits with padding and another 32 bits for the integer/float value (on 64 bits machines). It has the benefit that on 16 bits machines, the size will halve to 32 bits, as all members will reduce their size.

[kalvdans]

As a first step maybe use int32_t for the type to make sure we get 32 bits even on 16-bit platforms?

[rodarima]

Merged in #264