Impose the same large limit as the slower 2-pass blur.

Also renames "RRectShadow" to the much less ambiguous "SolidRRectBlur".

Before, a large enough blur sigma would result in nothing getting drawn. The Gaussian integral seems to be working fine against 16bit ops, so no adjustments were needed in the shader.

![Screenshot 2023-05-23 at 4 08 41 PM](https://github.com/flutter/engine/assets/919017/108e01ce-2cf1-4bc9-b51b-7f4ad7288fac)