[Impeller][GLES] SSBO support for Impeller GLES backend #193162

The Impeller GLES backend currently does not support SSBO:

If we wanted to add SSBO support to GLES, we'd need to add functionality in the GLES backend to query the device for SSBO support and update the GLES backend plumbing and binding code so that it is able to use SSBO.

#192962 adds SSBO-based UberSDF gradient rendering, which is more performant than the existing texture-based UberSDF gradient rendering. But it only works for backends which currently support SSBO (Metal, Vulkan). Windows uses GLES, so this does not currently improve performance on Windows. Adding SSBO support to the GLES backend will let this address Windows performance (#190401).