Reduce cost of resetting row attributes (#15497)

Performance of printing enwik8.txt at the following block sizes:
4KiB (printf): 54MB/s -> 54MB/s
128KiB (cat): 101MB/s -> 104MB/s

## Validation Steps Performed
This change is easily verifiable via review.

src/buffer/out/Row.cpp

@@ -123,11 +123,13 @@ LineRendition ROW::GetLineRendition() const noexcept
 // - Attr - The default attribute (color) to fill
 // Return Value:
 // - <none>
-void ROW::Reset(const TextAttribute& attr)
+void ROW::Reset(const TextAttribute& attr) noexcept
 {
     _charsHeap.reset();
     _chars = { _charsBuffer, _columnCount };
-    _attr = { _columnCount, attr };
+    // Constructing and then moving objects into place isn't free.
+    // Modifying the existing object is _much_ faster.
+    *_attr.runs().unsafe_shrink_to_size(1) = til::rle_pair{ attr, _columnCount };
     _lineRendition = LineRendition::SingleWidth;
     _wrapForced = false;
     _doubleBytePadded = false;

src/buffer/out/Row.hpp

@@ -124,7 +124,7 @@ class ROW final
     void SetLineRendition(const LineRendition lineRendition) noexcept;
     LineRendition GetLineRendition() const noexcept;
 
-    void Reset(const TextAttribute& attr);
+    void Reset(const TextAttribute& attr) noexcept;
     void TransferAttributes(const til::small_rle<TextAttribute, uint16_t, 1>& attr, til::CoordType newWidth);
     void CopyFrom(const ROW& source);
 

src/inc/til/rle.h

@@ -402,6 +402,11 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
             return _runs;
         }
 
+        container& runs() noexcept
+        {
+            return _runs;
+        }
+
         // Get the value at the position
         const_reference at(size_type position) const
         {

src/inc/til/small_vector.h

@@ -606,6 +606,24 @@ namespace til
             _capacity = capacity;
         }
 
+        // This is a very unsafe shortcut to free the buffer and get a direct
+        // hold to the _buffer. The caller can then fill it with `size` items.
+        [[nodiscard]] T* unsafe_shrink_to_size(size_t size) noexcept
+        {
+            assert(size <= N);
+
+            if (_capacity != N)
+            {
+                _deallocate(_data);
+            }
+
+            _data = &_buffer[0];
+            _capacity = N;
+            _size = size;
+
+            return &_buffer[0];
+        }
+
         void push_back(const T& value)
         {
             emplace_back(value);