[GLSL] Vertex shader ssbo emulation

Is it hard to implement read-only ssbo emulation via usampler2D/usamplerBuffer/uniformBuffer in a vertex shader if not supported? GLSL may not support ssbo in shaders other than fragment and compute.

Emulation example (via uniform buffer):

```
#define SSBO_SIZE (4 * 2)

buffer SSBO
{
float floats[2];
}

void main()
{
float f0 = floats[0];
float f1 = floats[1];
...
}

-->

uniform SSBO
{
uvec4 buf[(SSBO_SIZE + 15) / 16];
};

void main()
{
float f1 = uintBitsToFloat(buf[0].x);
float f2 = uintBitsToFloat(buf[0].y);
...
}
```

No files, tests, or entry points are named. Start by examining how vertex shaders and SSBOs are currently handled, then compare that with the issue's usampler2D, usamplerBuffer, and uniform-buffer emulation example; done means read-only SSBO access works in vertex shaders where native support is unavailable.

Written by the indexing model from the issue text.