fix TrianglesDrawer flush (SetLen)

gltriangles.go

@@ -10,7 +10,7 @@ import (
 // NewGLTriangles returns OpenGL triangles implemented using pixelgl.VertexSlice. A few notes.
 //
 // Triangles returned from this function support TrianglesPosition, TrianglesColor and
-// TrianglesTexture. If you need to support more, you can "override" Update and Append method.
+// TrianglesTexture. If you need to support more, you can "override" SetLen and Update method.
 //
 // Draw method simply draws the underlying pixelgl.VertexSlice. It needs to be called in the main
 // thread manually. Also, you need to take care of additional Target initialization or setting of

graphics.go

@@ -130,6 +130,7 @@ func (td *TrianglesDrawer) flush() {
 	td.dirty = false
 
 	for _, t := range td.tris {
+		t.SetLen(td.Len())
 		t.Update(td.Triangles)
 	}
 }