Statically check whether we are using the fields from the right GL version.

GLES 2.0 commands should not be using GLES 3.0 constants (unless added by extension).
The validator should be able to verify this is true.
I am thinking about something like this:

`
@if(Version.GLES20 || Version.EXT_draw_buffers)
GLint MaxDrawBuffers

@if(Version.GLES20)
cmd Foo() {
_ := ctx.Constants.MaxDrawBuffers // ok
}

@if(Version.GLES30)
cmd Bar() {
_ := ctx.Constants.MaxDrawBuffers // error
}
`

Think of the @if as #if blocks - would the code compile with #define GLES20 1 and #define GLES30 0?

Similarly, we should hide fields in the client if the @if says they are not applicable to the current version.