Enable vertex pulling (storage buffers + vertex id)

[floooh]

Now that GLES2/WebGL1 has been dropped it's time to look into the topic of "rendering without vertex buffers" again:

• don't require a vertex layout specification when creating a pipeline object
• d3d11: _sg_validate_shader_desc(): don't require a semantic name in the first vertex attribute
• don't require a vertex buffer to be bound for rendering
• add storage buffer type (e.g. indexable items)
• check that gl_VertexID is properly supported by sokol-shdc

PS: WebGL2 doesn't have storage buffer support either. It still should support rendering without vertex buffers though (e.g. pulling the data from a texture or uniform)

[floooh]

WIP vertex pulling example:

• C: https://github.com/floooh/sokol-samples/blob/storage-buffers/sapp/vertexpull-sapp.c
• GLSL: https://github.com/floooh/sokol-samples/blob/storage-buffers/sapp/vertexpull-sapp.glsl

'bufferless rendering' example:

• C: https://github.com/floooh/sokol-samples/blob/storage-buffers/sapp/triangle-bufferless-sapp.c
• GLSL: https://github.com/floooh/sokol-samples/blob/storage-buffers/sapp/triangle-bufferless-sapp.glsl

[floooh]

Fixed.