Update the maxPerStageDescriptorStorageBuffers from 4 to 16.
ANGLE requires at least 12 for GLES3.1 support, and 16 is a common
number among actual hardware so bumping to 16.

Bug: swiftshader:136
Change-Id: I2790a8cab197c773aa14d9c1b0fdcc5ca8356b25
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38068
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Tobin Ehlis <tobine@google.com>