Add support for older hardware

[derezzedex]

This PR aims to add support for older hardware, depends on grovesNL/glow#196.
Should close #4.

Changes

For this, this PR does a couple of things:

• Separate the pipeline into core and compatibility.
• Modify shaders based on context version.
• Change texture format based on context version.

I've also:

• Added Pipeline::get_max_texture_size (which had a TODO)
• Described the changes on CHANGELOG.md

Core and Compatibility

The main idea here is to separate the core pipeline, which is preferred and used on hardware that supports it and the compatibility pipeline which is the fallback, used on hardware that doesn't have the features needed for core.

Right now, the main thing separating them is the support for instancing, this means that core uses instancing while compatibility uses a vertex and index buffer.

Feedback

I think the separation between the core and compatibility could be improved to reduce code duplication and futher changes/maintenance easier. Any feedback and testing is appreciated.

[hecrj]

This is perfect! Great work 🎉

I have made a couple of small changes:

• Updated glutin to 0.28, as I wasn't able to run the examples with the older version.
• Updated glow to 0.11.1, which already contains the changes in Expose Version and add Context::version grovesNL/glow#196 🎉
• Updated the CHANGELOG to only mention the changes that are important to end-users.

The integration workflow is failing on the beta toolchain because of an issue on winit and some changes in the linking strategy of Rust (see rust-windowing/winit#2078).

In any case, I think we can 🚢 !