go v.1 - Update the Makefile to work with the current version of go

[flavius]

Update the Makefile to work with the current version of go

[flav@evolution shadergo]$ go run main.go 
go build _/tmp/shadergo/canvas: chdir /tmp/shadergo/canvas: no such file or directory
go build _/tmp/shadergo/file: chdir /tmp/shadergo/file: no such file or directory
go build _/tmp/shadergo/gyu3d: chdir /tmp/shadergo/gyu3d: no such file or directory
go build _/tmp/shadergo/shader: chdir /tmp/shadergo/shader: no such file or directory

[ylz-at]

Please update to work with go v1.15.

• file-> use os.File
• color -> use color.Color
• err -> use error
• fix some interface changes such as CanvasImage.Bounds should be added to implement image.Image Interface
• documentation such as readme and stuff
• use current go package, support go.mod