Make OwnerBatch::param public

Closes https://github.com/gfx-rs/gfx-rs/issues/368
Merge pull request #369 from bvssvni/pub-param

Make OwnerBatch::param public
Derived PartialEq for shader Stage
Merge pull request #370 from sectopod/master

Derived PartialEq for shader Stage
Check for errors after every draw command
Print more useful information on errors

The "if cfg!(not(ndebug))" mirrors the implementation of
debug_assert_eq!, so it shouldn't be any more expensive than the macro
itself.
Merge pull request #373 from hannobraun/error-checking

Improve error checking
Rename ErrorType to GlError

GlError is more specific and sounds nicer to me.
Simplify GlDevice::get_error
Move creation code for GlError out of GlContext

This makes GlError useful outside of GlContext.
Merge pull request #374 from hannobraun/gl-error-handling

Improve handling of OpenGL errors
Fix stack overflow when formatting ColorMask

The old implementation of ColorMask relied on itself for formatting,
leading to an infinite recursive loop and eventual stack overflow.

Thanks to @bjz for suggesting this approach.
Return TextureError, if TextureInfo is invalid
Improve error messages for TextureError
Merge pull request #372 from hannobraun/color-mask-format

Fix stack overflow when formatting ColorMask
Merge pull request #375 from hannobraun/invalid-textureinfo

Fail with error message, if TextureInfo is invalid
Update for glfw-rs changes
Merge pull request #377 from cmr/get_proc_address

Update for glfw-rs changes
Update README
Merge pull request #378 from lucidd/master

Update README