Remove `const_ptr_null_mut` feature attribute

Closes orbtk#89

See merge request redox-os/orbclient!60

This feature has been stabilized for some time.
The nightly compiler started throwing errors on these obsolete attributes.

Fix nightly build

See merge request redox-os/orbclient!59

Alloc doesn't reexport Vec in the crate root anymore.
https://github.com/rust-lang/rust/commit/3394fb7bb7f08c045f9a82bb92272418c855859d

[bugfix] blur algorithm and box shadow radius calculation

See merge request redox-os/orbclient!56

[bugfix] box shadow radius calculation.

[feature] box shadow

See merge request redox-os/orbclient!55

[feature] gaussian blur box

See merge request redox-os/orbclient!54

Add gitlab CI & tiny cleanup

See merge request redox-os/orbclient!53

Fix issue with edge-triggered orbital

fix getpixel()

Fix pixel() alpha blending , clean code.