Discard fragments with zero texture alpha in imesh_tex.frag.

This makes image requests that have an image with a hole in it
actually transparent, since otherwise the depth test would prevent
any geometry behind the request from being drawn.

res/shaders/imesh_tex.frag

@@ -9,5 +9,7 @@ uniform sampler2D texture;
 varying vec2 fragTex;
 
 void main() {
-    gl_FragColor = texture2D(texture, fragTex) * color;
+    vec4 texColor = texture2D(texture, fragTex);
+    if(texColor.a == 0.0) discard;
+    gl_FragColor = texColor * color;
 }