mesa/st: Don't use 4bits for GL_UNSIGNED_BYTE_3_3_2(_REV)

4bits and 3bits quantitization values differ significantly for
values other than 0 and 1.

Fixes piglit draw-pixels for softpipe/llvmpipe.

NOTE: Probably a candidate for stable branches.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>

src/mesa/state_tracker/st_cb_drawpixels.c

@@ -396,6 +396,8 @@ internal_format(struct gl_context *ctx, GLenum format, GLenum type)
 
          case GL_UNSIGNED_BYTE_3_3_2:
          case GL_UNSIGNED_BYTE_2_3_3_REV:
+            return GL_R3_G3_B2;
+
          case GL_UNSIGNED_SHORT_4_4_4_4:
          case GL_UNSIGNED_SHORT_4_4_4_4_REV:
             return GL_RGBA4;