st/mesa: Validate the state in st_readpixels.

The front renderbuffer of a framebuffer is usually added as needed when
glReadBuffer(GL_FRONT) is called.  When the call is followed by
glReadPixels, we should validate the state before reading from the
renderbuffer.

src/mesa/state_tracker/st_cb_readpixels.c

@@ -45,6 +45,7 @@
 
 #include "st_debug.h"
 #include "st_context.h"
+#include "st_atom.h"
 #include "st_cb_readpixels.h"
 #include "st_cb_fbo.h"
 #include "st_public.h"
@@ -352,6 +353,8 @@ st_readpixels(GLcontext *ctx, GLint x, GLint y, GLsizei width, GLsizei height,
    /* XXX convolution not done yet */
    assert((transferOps & IMAGE_CONVOLUTION_BIT) == 0);
 
+   st_validate_state(ctx->st);
+
    /* Do all needed clipping here, so that we can forget about it later */
    if (!_mesa_clip_readpixels(ctx, &x, &y, &width, &height, &clippedPacking)) {
       /* The ReadPixels transfer is totally outside the window bounds */