shader: Define special variables for textures __t%d

This eanbles to define texture0At to texture3At.

Updates #1193

internal/shader/expr.go

@@ -25,7 +25,7 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/shaderir"
 )
 
-var textureAtRe = regexp.MustCompile(`\Atexture(\d+)At\z`)
+var textureVariableRe = regexp.MustCompile(`\A__t(\d+)\z`)
 
 func (cs *compileState) parseExpr(block *block, expr ast.Expr) ([]shaderir.Expr, []shaderir.Type, []shaderir.Stmt, bool) {
 	switch e := expr.(type) {
@@ -220,12 +220,6 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr) ([]shaderir.Expr,
 				t = shaderir.Type{Main: shaderir.Vec3}
 			case shaderir.Texture2DF:
 				t = shaderir.Type{Main: shaderir.Vec4}
-				args = append([]shaderir.Expr{
-					{
-						Type:  shaderir.TextureVariable,
-						Index: callee.Index,
-					},
-				}, args...)
 			default:
 				t = argts[0]
 			}
@@ -359,13 +353,12 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr) ([]shaderir.Expr,
 				},
 			}, nil, nil, true
 		}
-		if m := textureAtRe.FindStringSubmatch(e.Name); m != nil {
+		if m := textureVariableRe.FindStringSubmatch(e.Name); m != nil {
 			i, _ := strconv.Atoi(m[1])
 			return []shaderir.Expr{
 				{
-					Type:        shaderir.BuiltinFuncExpr,
-					BuiltinFunc: shaderir.Texture2DF,
-					Index:       i, // Index is used as a texture ID later.
+					Type:  shaderir.TextureVariable,
+					Index: i,
 				},
 			}, nil, nil, true
 		}

shader.go

@@ -16,15 +16,17 @@ package ebiten
 
 import (
 	"bytes"
+	"fmt"
 	"go/parser"
 	"go/token"
+	"strings"
 
 	"github.com/hajimehoshi/ebiten/internal/buffered"
 	"github.com/hajimehoshi/ebiten/internal/graphics"
 	"github.com/hajimehoshi/ebiten/internal/shader"
 )
 
-const shaderSuffix = `
+var shaderSuffix = `
 var __viewportSize vec2
 
 func viewportSize() vec2 {
@@ -32,6 +34,20 @@ func viewportSize() vec2 {
 }
 `
 
+func init() {
+	// __t%d is a special variable for a texture variable.
+	// TODO: Add appropriate offsets for second and following images.
+
+	var fs []string
+	for i := 0; i < graphics.ShaderImageNum; i++ {
+		fs = append(fs, fmt.Sprintf(`func texture%[1]dAt(pos vec2) vec4 {
+	return texture2D(__t%[1]d, pos)
+}
+`, i))
+	}
+	shaderSuffix += "\n" + strings.Join(fs, "\n")
+}
+
 type Shader struct {
 	shader *buffered.Shader
 }