[epaper_spi] Refactoring (#11540)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

esphome/components/split_buffer/split_buffer.h

@@ -4,7 +4,13 @@
 #include <cstdlib>
 
 namespace esphome::split_buffer {
-
+/**
+ * A SplitBuffer allocates a large memory buffer potentially as multiple smaller buffers
+ * to facilitate allocation of large buffers on devices with fragmented memory spaces.
+ * Each sub-buffer is the same size, except for the last one which may be smaller.
+ * Standard array indexing using `[]` is possible on the buffer, but, since the buffer may not be contiguous in memory,
+ * there is no easy way to access the buffer as a single array, i.e. no `.data()` access like a vector.
+ */
 class SplitBuffer {
  public:
   SplitBuffer() = default;
@@ -19,13 +25,13 @@ class SplitBuffer {
   // Access operators
   uint8_t &operator[](size_t index);
   const uint8_t &operator[](size_t index) const;
+  void fill(uint8_t value) const;
 
   // Get the total length
   size_t size() const { return this->total_length_; }
 
   // Get buffer information
   size_t get_buffer_count() const { return this->buffer_count_; }
-  size_t get_buffer_size() const { return this->buffer_size_; }
 
   // Check if successfully initialized
   bool is_valid() const { return this->buffers_ != nullptr && this->buffer_count_ > 0; }