Change brightness effect to match Scratch 2.0 in 2D mode

[cwillisf]

I wasn't sure who would want to review a GLSL change so I just assigned a bunch of people. Feel free to unassign yourself if you're not interested :)

If you're unfamiliar with GLSL or just need a refresher, take a look at pages 3 & 4 of this PDF:
https://www.khronos.org/files/webgl/webgl-reference-card-1_0.pdf

Resolves

Resolves #142

Proposed Changes

Previously this repository's brightness effect matched the behavior of Scratch 2.0's 3D renderer by adjusting the value component of the HSV representation of a color. Now, it matches the behavior of Scratch 2.0's 2D renderer, which simply offsets (and clamps/saturates) the RGB values.

Note that to fully match Scratch 2.0's 2D renderer the brightness effect must happen after the color effect, not before.

Reason for Changes

It appears that most uses of the brightness effect in Scratch 2.0 are not paired with other graphical effects which cause the 3D renderer to activate, so matching the 2D renderer will likely cause fewer projects to display incorrectly in Scratch 3.0.

Test Coverage

Testing is easiest by comparing the Scratch 2.0 renderer against the scratch-render playground. See also #142 for example projects to test.

[cwillisf]

Note that adding ?w=1 to the URL of the diff view will tell GitHub's diff to ignore whitespace differences and make the diff much more readable for this change :)

[paulkaplan]

LGTM. It seems like we are bound to make someone upset with this change, I just can't tell which group is bigger (this change will re-break the original issue it is linked to #142)

But code LG!

[cwillisf]

After talking with both @paulkaplan and @rschamp I think the right move is to merge this for now and then consider more complicated solutions if we receive a lot of complaint.