sokol_gfx.h gl: fix max number of storage buffer bindings (fixes #1193)

[floooh]

This reduces the max occupied storage buffer bindslots in the GL backend from 16 to 8 (same as the max number of storage buffer bindings across all shader stages).

Requires a matching sokol-shdc update (floooh/sokol-tools#170), since previously sokol-shdc would assign fragment shader storage buffer bindings starting at slot 16 (now sokol-shdc simply uses the original binding 0..7).

TODO:

• fix any places in sokol-tools code comments and documentation which may refer to GL bindings from 0..15
• fix any places in sokol_gfx.h code comments which may refer to GL bindings from 0..15
• sokol-tools changelog
• sokol changelog
• add release-mode out-of-range check for glsl_binding_n