Refactor: Uniform optimization + javadoc

[capdevon]

This PR refactors the Uniform class in com.jme3.shader to improve code clarity, maintainability, and usage safety. Key changes include:

• Replacing wildcard and redundant imports with explicit class imports.
• Improving null checks, input validation, and variable naming for clearer intent.
• Unifying and simplifying handling of value assignment in setValue.
• Replacing manual hashCode and equals implementations with Objects.hash and Objects.equals.
• Adding and enhancing Javadoc comments for public methods and class-level documentation.
• Using meaningful constants for location checks and buffer allocation.
• Ensuring proper buffer cleanup in deleteNativeBuffers with explanatory comments.
• General code cleanup, minor bug fixes, and consistency improvements.

These updates make the Uniform class easier to understand, safer to use, and better documented for both users and maintainers.