Add configurable pixmap caching with LRU eviction

This commit implements transparent pixmap caching to improve performance
by reusing frequently allocated pixmaps with identical format and
dimensions.

Cache features:
- Automatic operation in twin_pixmap_create/destroy
- LRU eviction policy respecting reference counts to prevent use-after-free
- Hash-based lookup with collision handling via eviction

Configuration options (added to Features menu):
- CONFIG_PIXMAP_CACHE: Enable/disable caching (default: y)
- CONFIG_PIXMAP_CACHE_SIZE: Entry count 4-32 (default: 8)
- CONFIG_PIXMAP_CACHE_MAX_KB: Memory limit 16-512KB (default: 64KB)

Author: jserv

configs/Kconfig

@@ -77,6 +77,32 @@ config SHADOW_BLUR
     range 1 10
     depends on DROP_SHADOW
 
+config PIXMAP_CACHE
+    bool "Enable pixmap caching"
+    default y
+    help
+      Enable caching of pixmaps to improve performance by reusing
+      frequently allocated pixmaps. This reduces memory allocation
+      overhead at the cost of slightly increased memory usage.
+
+config PIXMAP_CACHE_SIZE
+    int "Maximum number of cached pixmaps"
+    default 8
+    range 4 32
+    depends on PIXMAP_CACHE
+    help
+      Maximum number of pixmaps that can be stored in the cache.
+      Larger values may improve performance but use more memory.
+
+config PIXMAP_CACHE_MAX_KB
+    int "Maximum cache size in KB"
+    default 64
+    range 16 512
+    depends on PIXMAP_CACHE
+    help
+      Maximum total memory size for cached pixmaps in kilobytes.
+      The cache will evict entries to stay within this limit.
+
 endmenu
 
 menu "Image Loaders"