Refactor Gaussian blur API

• Changed the filter context creation API:
  • More intuitive function and arguments names
  • Kernel size parameters added
  • Intermediate size now automatically deduced from the kernel size parameters
• Changed the implementation API:
  • Unified separate kernel size APIs into one
  • Changed specification to allow future support for custom sigma values
• Simplified the HAL handler
• Modified unit tests and also added some new ones
• Other miscellaneous refactoring