Firstly...we have a feature request thread:
https://forum.thegamecreators.com/thread/212232
Secondly this feature is easily achieved with a sprite shader that checks the alpha value and discards the pixel if the alpha is lower than the value specified. Combine this with setting the sprite transparency mode to 0 and it already does exactly what you want.
EDIT: heres actual shader i used to do the same thing (it might not be optimal but it works fine
uniform sampler2D texture0;
varying mediump vec2 uvVarying;
varying mediump vec4 colorVarying;
uniform float Alpha; // shader constant to set the refernece alpha level - could use colourVarying.a if wanted??
void main()
{
gl_FragColor = texture2D(texture0, uvVarying) * colorVarying;
if (gl_FragColor.a<Alpha) discard; // throw away the pixel if is alpha isnt large enough
gl_FragColor.a = 1.0; // ensure the alpha that is drawn to the screen is fully opaque - no varying alpha in the final image
}
you can assign this shader to a sprite then set the reference alpha level and any pixel with alpha less than the reference is thrown away
