mesa: use unsigned types when performing bitshifting

Ensure unsigned integers are used instead of signed ones when performing
left bit shifts.

This has been detected by the Undefined Behaviour Sanitizer (UBSan).

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29772>

src/mesa/main/blend.c

@@ -1032,7 +1032,7 @@ _mesa_ColorMaski(GLuint buf, GLboolean red, GLboolean green,
 
    FLUSH_VERTICES(ctx, 0, GL_COLOR_BUFFER_BIT);
    ctx->NewDriverState |= ST_NEW_BLEND;
-   ctx->Color.ColorMask &= ~(0xf << (4 * buf));
+   ctx->Color.ColorMask &= ~(0xfu << (4 * buf));
    ctx->Color.ColorMask |= mask << (4 * buf);
    _mesa_update_allow_draw_out_of_order(ctx);
 }