Updates to use `float` type whenever it should be.

This commit fixes a lot of variables to use the `float` type. Basically all of these variables are used to draw something in nanovg, and which accepts `float` type in the first class.

This commit also updates the `Scroller` class to not cache rendering, or it would slow down the performance.
Improves the rendering performance in iOS.

This commit updates the `MOOpenGLView` class' `setupFramebuffer` method to improve the performance a bit. This is done by checking whether the `_framebuffer` property is already initialized. It also removes the `_stencilAndDepthRenderbuffer` property because it seems this would be unnecessary.
Renames function names for updating framebuffer.

This commit renames the `Widget::BeginRenderbufferUpdates()` and `Widget::EndRenderbufferUpdates()` methods to `Widget::BeginFramebufferUpdates()` and `Widget::EndFramebufferUpdates()`, respectively.