Add some functions

[Flash-nDie]

Hello.

PR Description

This PR adds several functions corresponding to the libvips C API.

Added Functions

• func (r *ImageRef) Background() ([]float64, error) : Retrieves the background colors.
• func (r *ImageRef) Subtract(in2 *ImageRef) error : Performs image subtraction.
• func (r *ImageRef) Abs() error : Applies the absolute value operation.
• func (r *ImageRef) Project() (*ImageRef, *ImageRef, error): Executes the project operation.

Modified Function

The text() function has been revised:

• The original implementation is no longer used.
• Since text() belongs to the create operations, it has been moved to create.c.
• func Text(params *TextParams) (*ImageRef, error) : Create text image.

[tonimelisma]

Thank you so much @Flash-nDie!