WebP/Lossy: Reduce size of MacroblockMetadata from 80 to 20 bytes

For a 1024x1024 image, saves about a quarter MB of memory use while
decoding (compared to the decompressed image data itself needing
4 MiB).  Not a huge win, but also very easy to do, so might as well.

No behavior change, no measurable performance impact.

Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp

@@ -405,7 +405,7 @@ ErrorOr<u8> tree_decode(BooleanDecoder& decoder, ReadonlySpan<TreeIndex> tree, R
 struct MacroblockMetadata {
     // https://datatracker.ietf.org/doc/html/rfc6386#section-10 "Segment-Based Feature Adjustments"
     // Read only if `update_mb_segmentation_map` is set.
-    int segment_id { 0 }; // 0, 1, 2, or 3. Fits in two bits.
+    u8 segment_id { 0 }; // 0, 1, 2, or 3. Fits in two bits.
 
     // https://datatracker.ietf.org/doc/html/rfc6386#section-11.1 "mb_skip_coeff"
     bool skip_coefficients { false };

Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossyTables.h

@@ -22,7 +22,7 @@ const TreeIndex MACROBLOCK_SEGMENT_TREE[2 * (4 - 1)] = {
 
 // https://datatracker.ietf.org/doc/html/rfc6386#section-8.2 "Tree Coding Example"
 // Repeated in https://datatracker.ietf.org/doc/html/rfc6386#section-11.2 "Luma Modes"
-enum IntraMacroblockMode {
+enum IntraMacroblockMode : u8 {
     DC_PRED,               /* predict DC using row above and column to the left */
     V_PRED,                /* predict rows using row above */
     H_PRED,                /* predict columns using column to the left */
@@ -45,7 +45,7 @@ static TreeIndex constexpr KEYFRAME_YMODE_TREE[2 * (num_ymodes - 1)] = {
 static Prob constexpr KEYFRAME_YMODE_PROBABILITIES[num_ymodes - 1] = { 145, 156, 163, 128 };
 
 // https://datatracker.ietf.org/doc/html/rfc6386#section-11.2 "Luma Modes"
-enum IntraBlockMode {
+enum IntraBlockMode : u8 {
     B_DC_PRED, /* predict DC using row above and column
                   to the left */
     B_TM_PRED, /* propagate second differences a la