gpu/internal: fix AccessBits to be a bitset and support read+write

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/internal/driver/driver.go

@@ -170,7 +170,7 @@ const (
 )
 
 const (
-	AccessRead AccessBits = 1 + iota
+	AccessRead AccessBits = 1 << iota
 	AccessWrite
 )
 

gpu/internal/opengl/opengl.go

@@ -785,6 +785,8 @@ func (b *Backend) BindImageTexture(unit int, tex driver.Texture, access driver.A
 		acc = gl.WRITE_ONLY
 	case driver.AccessRead:
 		acc = gl.READ_ONLY
+	case driver.AccessRead | driver.AccessWrite:
+		acc = gl.READ_WRITE
 	default:
 		panic("unsupported access bits")
 	}