Make Framebuffer match alpha settings of the parent context

[davepagurek]

Fixes #5

In p5 1.4.1, WebGL graphics/canvases default to not having an alpha channel to make alpha blending less confusing by default (thanks @SableRaf for finding this!) Previously, p5.Framebuffer would always have an alpha channel, leading to unexpected results in p5 1.4.1.

After this change:

• The framebuffer will create an alpha channel if and only if the WebGL context it's attached to has one
• The example sketch calls background(255) after clear(), since the behaviour of clear() looks different from v1.4.0 to v1.4.1 if you don't also set a background color