Fix typo in the illegal 3D texture assert logic. We care about catching
arrayed 3D textures or 3D shadow textures, not regular 3D textures.

- Minor perf improvement.

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Co-Authored-By: Mat M. <mathew1800@gmail.com>

Layered framebuffer attachments is a feature that allows applications to
write attach layered textures to a single attachment. What layer the
fragments are written to is decided from the shader using gl_Layer.

SPIR-V's Layer is GLSL's gl_Layer. It lets the application choose from a
shader stage (vertex, tessellation or geometry) which framebuffer layer
write the output fragments to.