Add BitmapContext extension methods for direct drawing operations

[Copilot]

This PR adds extension methods for the BitmapContext class that allow users to perform drawing operations directly on a BitmapContext instead of having to go through the WriteableBitmap. This enables more efficient code when doing multiple drawing operations since the BitmapContext only needs to be created once.

Key changes

• Created a new BitmapContextExtensions class with extension methods that mirror existing WriteableBitmap extension methods
• Implemented drawing methods: lines, rectangles, ellipses
• Implemented fill methods: rectangles, ellipses
• Implemented blit operations for copying pixels between contexts
• Implemented transform operations (flip)
• Added the class to all relevant project files

Usage example

Before, users needed to use WriteableBitmap methods that create/dispose contexts internally:

var bmp = new WriteableBitmap(500, 500);
// Each call creates and disposes a BitmapContext internally
bmp.DrawLine(10, 10, 100, 100, Colors.Red);
bmp.DrawRectangle(50, 50, 150, 150, Colors.Blue);
bmp.FillEllipse(200, 200, 300, 300, Colors.Green);

Now, users can reuse the same context for multiple operations:

var bmp = new WriteableBitmap(500, 500);
// Create the context once
using(var ctx = bmp.GetBitmapContext())
{
    // Multiple drawing operations using the same context
    ctx.DrawLine(10, 10, 100, 100, Colors.Red);
    ctx.DrawRectangle(50, 50, 150, 150, Colors.Blue);
    ctx.FillEllipse(200, 200, 300, 300, Colors.Green);
}
// Context is disposed only once

This provides better performance when doing many consecutive drawing operations, especially in loops, and also allows users to perform drawing operations from multiple threads.

Fixes #92.

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