mesa: add new signed rgba texture format

This is a (partial) backport of the signed texture format support in OGL 3.1.
Since it wasn't promoted from an existing extension roll our own.

src/mesa/state_tracker/st_cb_readpixels.c

@@ -486,7 +486,7 @@ st_readpixels(GLcontext *ctx, GLint x, GLint y, GLsizei width, GLsizei height,
             df += dfStride;
             if (!dfStride) {
                _mesa_pack_rgba_span_float(ctx, width, temp, format, type, dst,
-                                          &clippedPacking, transferOps);
+                                          &clippedPacking, transferOps, GL_FALSE);
                dst += dstStride;
             }
          }