Transparent camera causes unnecessary draw calls

In #3459, FlxCamera.fill() was changed to always fill the camera, even if the alpha is 0. As a result, even though this fill rect is entirely invisible, it still gets pushed and drawn by OpenGL, which we can see by using an OpenGL debugger like RenderDoc:

Here's a replay of the draw process, in video form to better illustrate my point. Notice how the first glDrawElements() call doesn't change anything visually (because it's entirely transparent).

https://github.com/user-attachments/assets/777e392a-9e63-4008-8abf-baccd3352d11

The alpha check was added back in 2013 in https://github.com/HaxeFlixel/flixel/commit/5dde99f83558a679e11e1d756a26305046f0318e with the intention of solving performance related issues mentioned in https://github.com/HaxeFlixel/flixel/issues/502. I believe we should re-add the check, and the memory leak from #3459 should be investigated further.

Start at FlxCamera.fill() and review the change from #3459 alongside commit 5dde99f83558a679e11e1d756a26305046f0318e. Verify that an alpha-zero fill no longer produces a draw call, then investigate whether the memory leak from #3459 is addressed or requires separate follow-up.