Function pointers returned through eglGetProcAddress should have a
GL_APIENTRY calling convention to avoid mismatches with their
declarations.

Bug: b/140700303
Change-Id: I706ca788e54a897a66cfdf5ba959ec6e3dc718ee
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36228
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>