Fix dcraw reading of black level for Sony

Don't set black level from tiff_bps if a black level is in the metadata.

Remove the Sony ILCE-7M3 black level from camconst.json. The black level
is in the metadata (512 for ISOs < 128000, 1024 for ISOs >= 128000).

rtengine/dcraw.h

@@ -174,6 +174,12 @@ protected:
         unsigned sub_frame_shot_select;
     };
 
+    struct SonyMeta
+    {
+        /// SR2SubIFD black levels tag 0x7310 exists.
+        bool sr2subifd_black2 = false;
+    };
+
     int fuji_total_lines, fuji_total_blocks, fuji_block_width, fuji_bits, fuji_raw_type, fuji_lossless;
 
     ushort raw_height, raw_width, height, width, top_margin, left_margin;
@@ -201,6 +207,7 @@ protected:
     std::string RT_software;
     double RT_baseline_exposure;
     struct MergedPixelshift merged_pixelshift;
+    struct SonyMeta sony_meta;
 
 public:
     struct CanonCR3Data {