Implement EGL_EXT_client_extensions.
Implement EGL_EXT_client_extensions.

BUG=18314459

Change-Id: I0f2e0391655714920c6f9f082bf59a59d02b7cd2
Reviewed-on: https://swiftshader-review.googlesource.com/1420
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader