Fix shader warning for macOS Catalina (#30)
Fix shader warning for macOS Catalina (#30)

In Catalina, shader creation will fail because of the following issue:

```
Compilation succeeded with:

program_source:22:21: warning: writable resources in non-void vertex function
vertex VertexOutput vertFunc2(unsigned int vID[[vertex_id]], device Vertex *pos [[buffer(0)]])
                    ^
program_source:22:77: note: writable buffer defined here
vertex VertexOutput vertFunc2(unsigned int vID[[vertex_id]], device Vertex *pos [[buffer(0)]])
                                                                            ^
```

Adding const to ensure the buffer is read-only.
emoon/minifb,emoon/minifb