GL_
Per-channel sampler swizzle masks (GL 3.3+ / GLES 3.0+ / WebGL2). Setting GL_TEXTURE_SWIZZLE_A = GL_ONE on a texture makes the sampler return alpha=1.0 regardless of the actual texture bytes — useful for video frames where the decoder leaves the X byte undefined and we'd otherwise have to force-fill alpha on every frame on the CPU.