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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[reneschulte]

@epsi1on Can you take a look if these are the wrappers and interfaces you had in mind?

[epsi1on]

Seems yes, but not completely.
What i had in mind was to simply replace all extensions of WritebaleBitmap into extensions of BitmapContext. (as stated in #76 and maybe other issues).

Although the BitmapContext does not allocated every time bmp.GetBitmapContext() is called (because a caching mechanism is there), but this would make it a little more robust I think.

If you are OK with this idea, I can make a pull request.

Thanks

[epsi1on]

Instead of replacing all extension methods of WriteableBitmap, there could be an identical copy which accepts BitmapContext instead:

original code:

public static void DrawLineAa(this WriteableBitmap bmp, int x1, int y1, int x2, int y2, int color, Rect? clipRect = null)
{
    //actual pixel manipulation code here
}

new code:

public static void DrawLineAa(this WriteableBitmap bmp, int x1, int y1, int x2, int y2, int color, Rect? clipRect = null)
{
    using(var ctx = bmp.GetBitmapContext())
        DrawLineAa(ctx,x1,y1,x2,y2,color,clipRect);
}

public static void DrawLineAa(this BitmapContext ctx, int x1, int y1, int x2, int y2, int color, Rect? clipRect = null)
{
    //actual pixel manipulation code here
}