Add PNG_IMAGES_PATCH for inline image previews

Port of suckless.org png_images patch into flexipatch build.
Adds -ip (image prefix) and -is (image size) flags.
Items prefixed with PNG_IMAGE: render inline PNG thumbnails.
Requires libspng.

Migrate to dmenu-flexipatch

Enables preprocessor-based patch management with the following patches:
- alpha: Transparency support
- border: Window border
- caseinsensitive: Case-insensitive matching by default
- center: Centered dmenu window
- ctrl_v_to_paste: Ctrl+V paste support
- fuzzymatch: Fuzzy matching
- highlight: Highlight matched text
- line_height: Configurable line height
- mouse_support: Mouse interaction
- numbers: Show item count
- password: Password input mode
- rejectnomatch: Reject non-matching input
- xresources: Runtime color configuration

Custom settings:
- Font: JetBrainsMono Nerd Font:size=14
- Colors: #2a4055 fg, #9CC2EC bg, #DEC292 selection
- Center enabled by default
- Border width: 2px

Initial commit: dmenu with center patch

My build of dmenu based on BreadOnPenguins' fork with center and xresources patches.

Features:
- Center patch (-c flag)
- Vertical listing (-l flag)
- Xresources/pywal support
- Alpha transparency
- Password mode (-P flag)
- Mouse support