added checks that LoadGL is called before anything else is done

canvas.go

@@ -102,6 +102,9 @@ const (
 // the origin, since GL uses the bottom left coordinate, the
 // coordinates given here also use the bottom left as origin
 func New(x, y, w, h int) *Canvas {
+	if gli == nil {
+		panic("LoadGL must be called before a canvas can be created")
+	}
 	cv := &Canvas{stateStack: make([]drawState, 0, 20)}
 	cv.SetBounds(x, y, w, h)
 	cv.state.lineWidth = 1

gradients.go

@@ -40,6 +40,9 @@ type gradientStop struct {
 // the coordinates from where to where the gradient
 // will apply on the canvas
 func NewLinearGradient(x0, y0, x1, y1 float64) *LinearGradient {
+	if gli == nil {
+		panic("LoadGL must be called before gradients can be created")
+	}
 	lg := &LinearGradient{gradient: gradient{from: vec{x0, y0}, to: vec{x1, y1}, opaque: true}}
 	gli.GenTextures(1, &lg.tex)
 	gli.ActiveTexture(gl_TEXTURE0)
@@ -61,6 +64,9 @@ func NewLinearGradient(x0, y0, x1, y1 float64) *LinearGradient {
 // gradient will apply from the first to the second
 // circle
 func NewRadialGradient(x0, y0, r0, x1, y1, r1 float64) *RadialGradient {
+	if gli == nil {
+		panic("LoadGL must be called before gradients can be created")
+	}
 	rg := &RadialGradient{gradient: gradient{from: vec{x0, y0}, to: vec{x1, y1}, opaque: true}, radFrom: r0, radTo: r1}
 	gli.GenTextures(1, &rg.tex)
 	gli.ActiveTexture(gl_TEXTURE0)

images.go

@@ -24,6 +24,9 @@ var images = make(map[string]*Image)
 // string. If you want the canvas package to load the image, make sure you
 // import the required format packages
 func LoadImage(src interface{}) (*Image, error) {
+	if gli == nil {
+		panic("LoadGL must be called before images can be loaded")
+	}
 	var img *Image
 	var err error
 	switch v := src.(type) {

text.go

@@ -30,6 +30,9 @@ var defaultFont *Font
 // LoadFont loads a font and returns the result. The font
 // can be a file name or a byte slice in TTF format
 func LoadFont(src interface{}) (*Font, error) {
+	if gli == nil {
+		panic("LoadGL must be called before fonts can be loaded")
+	}
 	var f *Font
 	switch v := src.(type) {
 	case *truetype.Font: