mesa: Allow NULL read/draw in complete FBOs in ARB_ES2_compatibility.

From the ARB_ES2_compatibility spec:

    "(8) How should we handle draw buffer completeness?

    RESOLVED: Remove draw/readbuffer completeness checks, and treat
    drawbuffers referring to missing attachments as if they were NONE."

Fixes arb_es2_compatibility-drawbuffers when the short-circuit for
ARB_ES2_compatibility in the previous commit is dropped.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/fbobject.c

@@ -795,7 +795,7 @@ _mesa_test_framebuffer_completeness(struct gl_context *ctx,
    }
 
 #if FEATURE_GL
-   if (ctx->API == API_OPENGL) {
+   if (ctx->API == API_OPENGL && !ctx->Extensions.ARB_ES2_compatibility) {
       /* Check that all DrawBuffers are present */
       for (j = 0; j < ctx->Const.MaxDrawBuffers; j++) {
 	 if (fb->ColorDrawBuffer[j] != GL_NONE) {