bindtexture: allow to set a texture on another texture unit

kivy/graphics/context_instructions.pxd

@@ -13,11 +13,11 @@ cdef class Color(ContextInstruction):
     cdef void apply(self)
 
 cdef class BindTexture(ContextInstruction):
+    cdef int _index
     cdef str _source
     cdef Texture _texture
     cdef void apply(self)
 
-
 cdef class PushMatrix(ContextInstruction):
     cdef void apply(self)
 

kivy/graphics/context_instructions.pyx

@@ -200,9 +200,11 @@ cdef class BindTexture(ContextInstruction):
         if self.source is None:
             self.texture = kwargs.get('texture', None)
 
+        self.index = kwargs.get('index', 0)
+
     cdef void apply(self):
         cdef RenderContext context = self.get_context()
-        context.set_texture(0, self._texture)
+        context.set_texture(self._index, self._texture)
 
     property texture:
         def __get__(self):
@@ -212,6 +214,15 @@ cdef class BindTexture(ContextInstruction):
                 texture = get_default_texture()
             self._texture = texture
 
+    property index:
+        def __get__(self):
+            return self._index
+        def __set__(self, int index):
+            if self._index == index:
+                return
+            self._index = index
+            self.flag_update()
+
     property source:
         '''Set/get the source (filename) to load for texture.
         '''