meta: Mask Stencil.Clear against stencilMax in _mesa_meta_Clear

This fixes incorrect behaviour when the stencil clear value exceeds
the size of the stencil buffer, for example, when set with:

glClearStencil (~1); /* Set a bit pattern of 111...11111110 */
glClear (GL_STENCIL_BUFFER_BIT);

The clear value needs to be masked by the value 2^m - 1, where m is the
number of bits in the stencil buffer. Previously, we passed the value
masked with 0x7fffffff to _mesa_StencilFuncSeparate which then clamps,
NOT masks the value to the range 0 to 2^m - 1.

The result would be clearing the stencil buffer to 0xff, rather than 0xfe.

Signed-off-by: Peter Clifton <pcjc2@cam.ac.uk>
Signed-off-by: Brian Paul <brianp@vmware.com>

src/mesa/drivers/common/meta.c

@@ -1398,6 +1398,7 @@ _mesa_meta_Clear(struct gl_context *ctx, GLbitfield buffers)
    struct vertex verts[4];
    /* save all state but scissor, pixel pack/unpack */
    GLbitfield metaSave = META_ALL - META_SCISSOR - META_PIXEL_STORE;
+   const GLuint stencilMax = (1 << ctx->DrawBuffer->Visual.stencilBits) - 1;
 
    if (buffers & BUFFER_BITS_COLOR) {
       /* if clearing color buffers, don't save/restore colormask */
@@ -1453,7 +1454,7 @@ _mesa_meta_Clear(struct gl_context *ctx, GLbitfield buffers)
       _mesa_StencilOpSeparate(GL_FRONT_AND_BACK,
                               GL_REPLACE, GL_REPLACE, GL_REPLACE);
       _mesa_StencilFuncSeparate(GL_FRONT_AND_BACK, GL_ALWAYS,
-                                ctx->Stencil.Clear & 0x7fffffff,
+                                ctx->Stencil.Clear & stencilMax,
                                 ctx->Stencil.WriteMask[0]);
    }
    else {