Check for both points when moving within viewport

[kelindar]

This pull request fixes a bug when the update isn't correctly fired when entering the viewport from one side. This is due to the contains check that wasn't checking properly for both before/after points.

Simplification and Refactoring:

• Removed redundant at parameter in writeTile, mergeTile, Add, Del, and Move methods in grid.go. [1] [2] [3] [4] [5]
• Simplified the Observers method in Tile to improve readability and efficiency.
• Removed redundant at and locs parameters in Notify1, Notify2, Each1, and Each2 methods in view.go. [1] [2]

Test Suite Enhancements:

• Added a new test TestView_Updates to cover various edge cases for the Move method in view_test.go.
• Removed commented-out test TestObservers to clean up the test suite.