fix(core): apply drawing buffer resize immediately for WebGL

[nokonoko1203]

Background

Commit 939287e ("Fix flicker when resizing CanvasContext") deferred the actual canvas resize from setDrawingBufferSize() to getCurrentFramebuffer() to avoid flicker caused by clearing the drawing buffer on resize.

However, on WebGL this deferred resize never fires. When beginRenderPass({framebuffer: null}) is called, the two backends behave differently.

WebGPU (webgpu-render-pass.ts) — calls getCurrentFramebuffer():

const framebuffer = (props.framebuffer as WebGPUFramebuffer) || device.getCanvasContext().getCurrentFramebuffer();

WebGL (webgl-render-pass.ts) — only calls getDrawingBufferSize():

const [width, height] = device.getDefaultCanvasContext().getDrawingBufferSize();
viewport = [0, 0, width, height];

As a result, on WebGL canvas.width/canvas.height are never updated from their initial values, while drawingBufferWidth/drawingBufferHeight are correctly set by ResizeObserver. The viewport is set to the intended size but the actual drawing buffer remains at the initial canvas size, causing a resolution mismatch.

Change List

• Call _resizeDrawingBufferIfNeeded() immediately after setting _needsDrawingBufferResize = true in setDrawingBufferSize()
• The deferred path in getCurrentFramebuffer() becomes a harmless no-op (flag is already cleared)