Fix extension checks for EGL external image tests

Fix extension checks for EGL external image tests

Commit 2090c97 extended dEQP-EGL.functional.image.modify.* with new
actions to test sampling from an external image. The actions implement
getRequiredExtension, so they were likely intended to be skipped if
GL_OES_EGL_image_external is not available.

However, extension checks are only performed for actions invoked through
ImageFormatCase. These actions are invoked one level down the call
stack, through RenderTryAll.

The result is that 22 tests in dEQP-EGL.functional.image.modify.* fail
during shader compilation if the extension is not available.

This commit adds an extension check to RenderTryAll::invokeGLES. Actions
will be skipped if the check fails. This is probably what was intended
in the first place, since RenderTryAll ignores NotSupported results.

Affects: dEQP-EGL.functional.image.modify.*

Components: EGL

Change-Id: Id1106d69fbe71d39695fc32513d80d23f1e94b40