Terrain block rendering does not use nearest-neighbor texture sampling

The bug

All textures on blocks' faces handled by the terrain shaders are sampled incorrectly, which leads to obvious interpolated pixel edges. This seems to be the case on all devices and in 25w44a and later.

For example, it doesn't apply to:

• Blocks in the GUI

• Entities

• Block entities

Steps to reproduce

1. Take a screenshot.

2. Zoom in to the image using a viewer with no filtering, i.e., with nearest-neighbor scaling.

3. Look at the edge between texture pixels within any block face, which will be blurry if the bug is present, and make sure to look inside the block face, not on its boundary.

Expected behavior

Performing these steps in versions prior to 25w44a, one would observe no blurred edges.

Additional notes

This issue dates back to 25w44a, which introduced anisotropic filtering. It is present regardless of the mipmap or anisotropic filtering settings. However, it might be related to the fix for MC-300642.

The handling of texelOffset and texelCenter may be problematic. In 25w45a, this alternate implementation of sampleNearest seems to work well at short and long distances but has not been thoroughly tested for potential issues:

vec4 sampleNearest(sampler2D sampler, vec2 uv, vec2 pixelSize) {
	// Figure out how big a texel is on the screen, more or less
	vec2 du = dFdx(uv);
	vec2 dv = dFdy(uv);

	// Convert our UV back up to texel coordinates and find out how far over we are from the center of each pixel
	vec2 uvTexelCoords = uv / pixelSize;
	vec2 texelCenter = round(uvTexelCoords + 0.5f) - 0.5f;
	vec2 texelOffset = uvTexelCoords - texelCenter;

	// Move our offset closer to the texel center based on texel size on screen
	texelOffset = clamp(texelOffset * pixelSize, -abs(du) * 0.5 - abs(dv) * 0.5, abs(du) * 0.5 + abs(dv) * 0.5);

	uv = (texelCenter) * pixelSize + texelOffset;
	return textureGrad(sampler, uv, du, dv);
}

The corresponding changes have also been tested on the latest snapshot and seem to work.