Avoid creating new List<RectangleI> when no texture uploads are required

[peppy]

This was triggering on every texture bind operation. Oops.

Loading song select (from an already warmed up glyph cache):

before	after

[smoogipoo]

This should also be applied to GLTexture:

osu-framework/osu.Framework/Graphics/OpenGL/Textures/GLTexture.cs

Lines 197 to 216 in 9f689c5

	public unsafe bool Upload()
	{
	if (!Available)
	return false;
	// We should never run raw OGL calls on another thread than the main thread due to race conditions.
	ThreadSafety.EnsureDrawThread();
	List<RectangleI> uploadedRegions = new List<RectangleI>();
	while (tryGetNextUpload(out ITextureUpload upload))
	{
	using (upload)
	{
	fixed (Rgba32* ptr = upload.Data)
	DoUpload(upload, (IntPtr)ptr);
	uploadedRegions.Add(upload.Bounds);
	}
	}

[peppy]

Good catch, applied.