[orx-jumpflood] Make module multiplatform

orx-jumpflood/src/shaders/glsl/alpha-threshold.frag

@@ -0,0 +1,9 @@
+uniform sampler2D tex0;
+in vec2 v_texCoord0;
+uniform float threshold;
+out vec4 o_color;
+
+void main() {
+    float ref = step(threshold , texture(tex0, v_texCoord0).a);
+    o_color = vec4(ref, ref, ref, 1.0);
+}