Update GLModelStruct.h

Clips (alpha*255) to 0-255.   Clipping is already used in drawing events, so I added it where it was missing.  Now we are handling alpha the same way as other game development tools.

Whilst users could control it inside their games, some people who are newer or  using fade in / fade out effects might use fractional increments and might be causing spilling over some amounts above 1 or below 0, sometimes using certain fractions the problem might be inevitable, causing unwanted glitches before the next step, so this will avoid (alpha*255) going below 0 or above 255.