mesa: Use GLdouble for depthMax in final unpack conversions.

The final step of _mesa_unpack_depth_span is to take the temporary
GLfloat depth values and convert them to the desired format.  When
converting to GL_UNSIGNED_INTEGER with depthMax > 0xffffff, we use
double-precision math to avoid overflow and precision problems.

Or at least that's the idea.  Unfortunately

   GLdouble z = depthValues[i] * (GLfloat) depthMax;

actually causes single-precision multiplication, since both operands are
GLfloats.  Casting depthMax to GLdouble causes the scaling to be done
with double-precision math.

Fixes a regression in oglconform's depth-stencil basic.read.ds test
since c60ac7b17993d28af65b04f9bbbf3ee74c35358c, where the expected and
actual values differed slightly.  For example, 0xcfa7a6 vs. 0xcfa7a4.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49772
Signed-off-by: Kenneth Graunke <[email protected]>

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