feat: add SDL2::Renderer#read_pixels

[takaokouji]

Summary

Add SDL2::Renderer#read_pixels method that wraps SDL_RenderReadPixels.

This is a resubmission of #29, addressing the review feedback in #29 (comment).

Changes from #29

• Accepts SDL2::PixelFormat: The format parameter now accepts both SDL2::PixelFormat and Integer, using the existing uint32_for_format() helper (consistent with create_texture etc.)
• Respects format=0 semantics: Passing 0 uses the rendering target's native format as defined by SDL2, instead of overriding it with ARGB8888
• Sample program included: sample/test_read_pixels.rb demonstrates the method with BMP output

Usage

# Read the entire screen in ARGB8888 format
pixels = renderer.read_pixels(nil, SDL2::PixelFormat::ARGB8888)

# Read a specific area using the rendering target's native format
rect = SDL2::Rect.new(0, 0, 100, 100)
pixels = renderer.read_pixels(rect, 0)

Parameters

• rect — SDL2::Rect or nil (nil reads the entire target)
• format — SDL2::PixelFormat or Integer (0 to use the rendering target's format)

Returns

Raw pixel data as a binary String.

Closes #28