meta: set viewport and projection matrix in _mesa_meta_GenerateMipmap

This fixes mipmap levels being clipped to the last viewport.

Based on a patch submitted by Pierre Willenbrock <pierre@pirsoft.de>

src/mesa/drivers/common/meta.c

          break;
       }
 
+      assert(dstWidth == ctx->DrawBuffer->Width);
+      assert(dstHeight == ctx->DrawBuffer->Height);
+
+      /* setup viewport and matching projection matrix */
+      _mesa_set_viewport(ctx, 0, 0, dstWidth, dstHeight);
+      _mesa_MatrixMode(GL_PROJECTION);
+      _mesa_LoadIdentity();
+      _mesa_Ortho(0.0F, dstWidth, 0.0F, dstHeight, -1.0F, 1.0F);
+
       _mesa_DrawArrays(GL_TRIANGLE_FAN, 0, 4);
    }