Fix compile error in vertex shader

Fixes shader compile error when run on Windows 10 with
Intel(R) HD Graphics 630
GL_VERSION: 4.4.0 - Build 27.20.100.9664

src/com/persesgames/jogl/explosion/explShader.vert

@@ -16,7 +16,7 @@ varying vec4 varying_Color;
 void main(void) {
     mat4    uniform_Projection = mat4(1);
 
-    gl_PointSize = 15f;
+    gl_PointSize = 15.0f;
     gl_Position = uniform_Projection * attribute_Position; //vec4(attribute_Position.x, attribute_Position.y, 0, 1);
     varying_Color = attribute_Color;
 }