[pngdrift]

Description

Closes #23270.

Previously dp3 loaded its sources without extending them to vec4 before applying the vec3 swizzle.
For a FLOAT_2 attribute, this caused the swizzle to reference the Z component of a vec2, resulting in a wgpu shader validation error like this:

Shader validation error: Entry point main at Vertex is invalid
 = Expression [13] is invalid
 = Swizzle component Z is outside of vector size Bi

Testing

Added stage3d/agal_dp3_attribute_formats.

Checklist

• I, a human, have self-reviewed this PR and fully understand the changes within.
• I have made or updated tests where possible.
• All of my commits are properly scoped, compile successfully, and pass all tests.
• This PR does not make sense to split up into smaller PRs.
• An LLM was involved in the authoring of this code.