Add some comments

graphics/opengl/context.go

@@ -1,15 +1,14 @@
 package opengl
 
-import (
-	"github.com/hajimehoshi/go-ebiten/graphics"
-	"github.com/hajimehoshi/go-ebiten/graphics/matrix"
-)
-
 // #cgo LDFLAGS: -framework OpenGL
 //
 // #include <stdlib.h>
 // #include <OpenGL/gl.h>
 import "C"
+import (
+	"github.com/hajimehoshi/go-ebiten/graphics"
+	"github.com/hajimehoshi/go-ebiten/graphics/matrix"
+)
 
 func enableAlphaBlending() {
 	C.glEnable(C.GL_TEXTURE_2D)

graphics/opengl/shader/draw_texture.go

@@ -25,6 +25,7 @@ func DrawTexture(native NativeTexture, projectionMatrix [16]float32,
 	if len(quads) == 0 {
 		return
 	}
+	// TODO: Check performance
 	shaderProgram := use(projectionMatrix, geometryMatrix, colorMatrix)
 	defer C.glUseProgram(0)
 

graphics/opengl/shader/program.go

@@ -85,7 +85,8 @@ func getLocation(program C.GLuint, name string, qvType qualifierVariableType) C.
 	locationName := C.CString(name)
 	defer C.free(unsafe.Pointer(locationName))
 
-	location := C.GLint(-1)
+	const invalidLocation = -1
+	location := C.GLint(invalidLocation)
 
 	switch qvType {
 	case qualifierVariableTypeAttribute:
@@ -95,7 +96,7 @@ func getLocation(program C.GLuint, name string, qvType qualifierVariableType) C.
 	default:
 		panic("no reach")
 	}
-	if location == -1 {
+	if location == invalidLocation {
 		panic("glGetUniformLocation failed")
 	}
 	shaderLocationCache[qvType][name] = location
@@ -112,7 +113,8 @@ func getUniformLocation(program C.GLuint, name string) C.GLint {
 }
 
 func use(projectionMatrix [16]float32,
-	geometryMatrix matrix.Geometry, colorMatrix matrix.Color) C.GLuint {
+	geometryMatrix matrix.Geometry,
+	colorMatrix matrix.Color) C.GLuint {
 	programId := programRegular
 	if !colorMatrix.IsIdentity() {
 		programId = programColorMatrix