API - Style: Implement initial 'overflow: hidden' strategy using glScissorRect

[bryphe]

Issue: We need some very basic clipping to support scroll views and scrollable content.

This implements overflow: hidden in a very naive way - if a node has overflow: hidden set, we'll figure out the screen-space bounding box for the node, and then use the glScissorRect API to constrain child rendering inside this rectangle.

This works fine for elements that are not rotated, but will not give proper results when there is a rotation or shearing transform applied. But for now, this is at least reasonable to get us going and implementing elements that have bounded constraints.

In the future, it'd be worth pursuing additional options for overflow: hidden:

• Using the stencil buffer (https://github.com/bryphe/reason-glfw/issues/61)
• Using a render texture (https://github.com/bryphe/reason-glfw/issues/64 / Performance - Advanced: ContainerNode that uses a render texture #54 )

Sample I added to test:

TODO:

• Merge in High-DPI display clean-up #190
• Test native app in 2x DPI
• Test WebGL app in standard DPI
• Test WebGL app in 2x DPI