mesa: rename MaxTransformFeedbackSeparateAttribs to MaxTransformFeedbackBuffers

This is a cleanup for ARB_transform_feedback3, where
GL_MAX_TRANSFORM_FEEDBACK_BUFFERS is introduced for interleaved attribs and
has the same meaning as GL_MAX_.._SEPARATE_ATTRIBS for separate attribs.

Also, the maximum number of TFB buffers is reduced from 32 to 4, which makes
this patch useful even without the extension.
I don't know of any hardware which can do more than 4.

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

src/mesa/main/bufferobj.c

@@ -869,7 +869,7 @@ _mesa_DeleteBuffersARB(GLsizei n, const GLuint *ids)
          if (ctx->TransformFeedback.CurrentBuffer == bufObj) {
             _mesa_BindBufferARB( GL_TRANSFORM_FEEDBACK_BUFFER, 0 );
          }
-         for (j = 0; j < MAX_FEEDBACK_ATTRIBS; j++) {
+         for (j = 0; j < MAX_FEEDBACK_BUFFERS; j++) {
             if (ctx->TransformFeedback.CurrentObject->Buffers[j] == bufObj) {
                _mesa_BindBufferBase( GL_TRANSFORM_FEEDBACK_BUFFER, j, 0 );
             }