custom block render_method bugs

blend:

When using the blend render method, you can't see the texture behind the transparent pixels. 

Steps to Reproduce:

1. Prepare an image with transparent pixels and a custom block JSON using the "blend" render method;

2. Apply this image to the custom block;

3. Enter the game, hold the custom block and look at your skin through its transparent pixels, or place the block on the ground and look at its texture through its transparent pixels.

Observed Results:
The texture on the back cannot be seen through the transparent pixels.

alpha_test:

When using the "alpha_test" render method, the texture flickers when one of the dimensions of the geometry cube size is 0 and there are textures on both sides.

Steps to Reproduce:

1. Prepare an image and a custom block JSON using the "alpha_test" render method;

2. Give the block a custom geometry that contains any cube of size with a value of 0 on one axis, and then use textures for all the faces;

3. Enter the game, place the block on the ground, and move the perspective.

The geometry JSON example:

"cubes": [
  {
    "origin": [-5, 0, -6],
    "size": [12, 8, 0], //The third value is 0, which is a slice.
    "uv": {
      "north": {"uv": [2, 8], "uv_size": [12, 8]},
      "south": {"uv": [2, 8], "uv_size": [12, 8]}
    } //Texture on both sides.
  }
]

Observed Results:
As you move the view, you will notice that the texture of the custom cube is constantly flashing, as if the surfaces of two cubes are overlapping.