Start working on new the new effect stacking system

- CCEffectStack is now a container of CCEffects and does not just collapse
them down into a single new composite effect (though for now that functionality
remains).
- CCEffectNode now has a CCEffectStack instead of a single CCEffect
- CCEffectNode drawing iterates over the effects in its stack and, for each effect,
the passes within the current effect.

Now I need to hook up passing the results from the previous pass to the current
pass.
Split CCEffectBrightnessAndContrast to CCEffectBrightness and CCEffectContrast
Add helper function to remove a bunch of effect setup boilerplate.
Continue CCEffect stacking

The bulk of this change is the movement certain responsibilities from CCEffect subclasses and into
CCEffectNode. Before this, multi-pass effects were responsible for rendering the children of CCEffectNode
into an FBO as their first pass and then rendering the results of the last pass into the displayed frame buffer.
Now that we're connecting effects together, this responsibility can't fall on each effect since an one effect
might be taking its input from (or writing its output to) another.

CCEffectNode now renders its children into an FBO prior to the first effect's first pass and then draws the
results of the last effect's last pass into the displayed frame buffer. All CCEffects now have a sampler uniform
called cc_PreviousPassTexture that can be used for reading the results of the previous pass.

It is possible that we will want to give CCEffects the ability to override the pre-render and post-render steps
that are now owned by CCEffectNode but we don't have a use case for that yet.
Enable single brightness and single contrast effects and add contrast + brightness
Scale the brightness offset value by the pixel's alpha value

This gets the brightness adjustment into pre-multiplied alpha space.
Set the cc_MainTexture uniform equal to the texture at the beginning of the current effect

It had been the texture at the beginning of all effects but that's less useful. For the most part
an effect should really only depend on its input texture, the textures corresponding to intermediate
render passes, and any uniforms it declares. Reaching all the way back to the texture at the beginning
of an effect chain seems less meaningful to me though we may end up finding a use for this in the
future.
Convert the glow effect to support stacking

This involves the following:

- Removal of the u_sampler2 uniform which is no longer needed since corresponding texture
can be accessed via cc_PreviousPassTexture.
- Conversion of the GLSL to use cc_PreviousPassTexture.
- Removal of the first render pass since effects are no longer responsible for rendering CCEffectNode's
children into an FBO before the interesting work starts.
- Removal of the code from the last pass that drew the resulting image into the displayable frame buffer
since effect's are no longer responsible for that work either.
Add first pass at CCEffectPixellate
Cleanup pre-render / post-render comments
Fix determination of effect node size

Was MAX(width, width) should be MAX(width, height)
Add a test case for CCEffectPixellate
Merge branch 'develop' into thayer/effectStacking

Conflicts:
	cocos2d-ui-tests/tests/CCEffectsTest.m
	cocos2d/CCEffectGlow.m
	cocos2d/CCEffectNode.m
Remove duplicate ; character
Temporarily disable the composite effect test

This style of effect compositing is currently broken by the stacking changes. Disable it
for now while I fix it.