Shader target buffers no longer saved between frames

post_effect shader target buffers are no longer saved between frames.

To prove this point, I've attached a resource pack. This pack renders green when the previous frame contained data in the shader target buffer, and red when the buffer is not initialized (all zeros). 

Steps to reproduce:

• Launch the game in 1.21.4

• Apply the provided resource pack with fabulous graphics settings on

• Observe that the screen renders green (explanation for why below)

• Close the game and relaunch in 25w06a

• Apply the resource pack in the same way above

• Observe that the screen renders red (explanation for why below)

Explanation:

In the post_effect 'transparency.json', the buffer named 'buffer' is copied to a secondary temporary buffer named 'buffer_copy'. This 'buffer_copy' is handed to the 'update_buffer' shader with the sampler name 'InSampler'.

here is a sample of the 'transparency.json' post_effect demonstrating what was outlined above

{
    "targets": {
        "buffer": {
            "width": 2,
            "height": 7
        },
        "buffer_copy": {
            "width": 2,
            "height": 7
        },
        ... // hidden since its irrelevant to this explanation.
    },
    "passes": [
        {
            "program": "minecraft:test/copy", // custom "blit" shader to copy buffers of different sizes
            "inputs": [
                {
                    "sampler_name": "In",
                    "target": "buffer"
                }
            ],
            "output": "buffer_copy"
        },
        {
            "program": "minecraft:test/updatebuffer",
            "inputs": [
                {
                    "sampler_name": "In",
                    "target": "buffer_copy"
                }
            ],
            "output": "buffer"
        },
        ... // vanilla implementation and rendering of the shader buffer
    ]
}

As you can see, the contents of the 'buffer' target are copied to 'buffer_copy' and then used as an input for the 'update_buffer' shader pass. This pass outputs back to the original 'buffer' target.

below is the code present in the 'update_buffer.fsh' which utilizes the 'buffer_copy' as 'InSampler'

#version 150

uniform sampler2D InSampler;

in vec2 texCoord;

out vec4 fragColor;

void main() {
    vec4 lastColor = texture(InSampler, texCoord);

    if(lastColor == vec4(0))
        fragColor = vec4(1, 0, 0, 1);
    else
        fragColor = vec4(0, 1, 0, 1);
}

The shader code samples the color of the previous value stored in 'buffer_copy', and then passes this value back to the original 'buffer'. if that value is the default initialized value of vec4(0) then the if statement present in the code will set the screen color to red. Assuming that the buffer was updated to the color red in the last frame and the buffers are saved between frames, then one can also assume that the if statement on any following frame will fail, and the screen will be colored green.

Expected Result: When loading this resource pack in 25w02a and above, the shader is expected to behave the same as it did in 1.21.4. There were no changes stated in the snapshot changelog relating to post_effect shader passes, so one can expect the shader target buffers to remain continuous between frames.
Actual Result (Bug): The resource pack does not behave the same as it did in the prior game release 1.21.4, and the shader target buffer resets at the start of the shader post_effect pass.