Fix g3nshaders producint non-standard output

According to the [docs](https://golang.org/pkg/cmd/go/internal/generate/), generated go files must contain a line that satisfies the following regexp: `^// Code generated .* DO NOT EDIT\.$`. This allows automated tools (like github web interface or linters) to appropriately mark them as generated and ignore some checks that don't apply to those files.

tools/g3nshaders/main.go

@@ -40,7 +40,7 @@ const (
 // TEMPLATE is a Go template to generate the output file with the shaders' sources and
 // maps describing the include and shader names and programs shaders.
 //
-const TEMPLATE = `// File generated by G3NSHADERS. Do not edit.
+const TEMPLATE = `// Code generated by G3NSHADERS. DO NOT EDIT.
 // To regenerate this file install 'g3nshaders' and execute:
 // 'go generate' in this folder.