2D-Canvas: Improve Render performance

[kimkulling]

Improved drawRect method implementation for unfilled rectangles

The new implementation for unfilled rectangles in the drawRect method offers better control and clarity:

Each edge of the rectangle is now drawn explicitly, improving precision.
The use of a constant thickness enhances maintainability.
However, there's a potential optimization opportunity:

Consider creating a single DrawCmd for the entire unfilled rectangle instead of four separate commands. This could improve performance, especially when drawing many unfilled rectangles. Here's a potential approach:

if (!filled) {
    DrawCmd *drawCmd = alloc();
    drawCmd->NumVertices = 8;  // 4 corners, each used twice
    drawCmd->Vertices = new RenderVert[drawCmd->NumVertices];
    // ... set up vertices for all four corners ...
    
    drawCmd->NumIndices = 8;  // 4 lines, 2 indices each
    drawCmd->Indices = new ui16[drawCmd->NumIndices];
    // ... set up indices to draw 4 lines ...
    
    drawCmd->PrimType = PrimitiveType::LineList;
    mDrawCmdArray.add(drawCmd);
}

This approach would reduce memory allocations and draw calls, potentially improving performance.