Add noise, RTL support and other fixes

[joogps]

This pull request adds:

• A new noise parameter to the .glur() modifier that lets you specify an amount of noise for the blur effect. The noise is applied through an overlay blending mode.
• It also adds leading and trailing as notation options for the BlurDirection enum. These options are evaluated at runtime and depend on the user's locale.
• Additionally, this pull request uses the view's SwiftUI size to calculate the blur gradient instead of the texture size. This should lead to a more uniform effect across platforms and display densities.

It fixes #10.