Allow tweaking z buffer when rendering polygon, improve explosion render

shader/twoD/basicTweakZ.vert

@@ -0,0 +1,12 @@
+#version 430 core
+layout (location = 0) in vec4 position;
+layout (location = 1) in vec4 color;
+layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
+out vec4 vColor;
+void main()
+{
+  vec4 posxy = theMat * vec4(position.xyz,1);
+  vec4 posz  = theMat * vec4(position.xyw,1);
+  gl_Position = vec4(posxy.xy,posz.z,1);
+  vColor = color;
+}