blur: add noise in perceptual space

The previous implementation added noise in linear space, which resulted in
the effect becoming more pronounced on black backgrounds. This patch
changes the process to be applied in perceptual space, by making the noise
addition pass a separate draw call and disabling GL_FRAMEBUFFER_SRGB during
that.

After this change, noise will look much more suppressed and almost never
grainy. This change also changes the range of the noise from
[-strength..strength) to [0..strength), as blending can only be either
additive or subtractive. As a result, users might need to ramp up their
noise parameter after this change.

v2: Add more explanation around the draw call.
v3: Fix noise not fading out with the fade out effect.
v4: Restore an accidentally removed comment.
v5: Add CCBUG.
v6: Rebase.
v7: Fix a formatting issue.

CCBUG: 409620