Add SDL3 GPU backend (Vulkan/Metal) with OpenGL fallback

tools/shaders/compile_shaders.cmake

@@ -0,0 +1,70 @@
+# compile_shaders.cmake — invoked via `cmake -P`.
+#
+# Required cache vars:
+#   GLSLC       — path to the glslc executable.
+#   SHADERS_DIR — path to the shaders/ directory.
+#
+# Walks SHADERS_DIR for:
+#   _common/passthrough.vk.glsl         -> _common/passthrough.vert.spv
+#   <name>/<name>.vk.glsl               -> <name>/<name>.frag.spv
+#
+# Shaders are recompiled only if the .spv is missing or older than its source.
+
+if(NOT GLSLC)
+    message(FATAL_ERROR "compile_shaders.cmake: GLSLC not provided")
+endif()
+if(NOT SHADERS_DIR)
+    message(FATAL_ERROR "compile_shaders.cmake: SHADERS_DIR not provided")
+endif()
+if(NOT EXISTS "${SHADERS_DIR}")
+    message(FATAL_ERROR "compile_shaders.cmake: SHADERS_DIR does not exist: ${SHADERS_DIR}")
+endif()
+
+function(_compile_to_spv SOURCE STAGE OUTPUT)
+    if(EXISTS "${OUTPUT}")
+        file(TIMESTAMP "${SOURCE}" SRC_T "%s")
+        file(TIMESTAMP "${OUTPUT}" OUT_T "%s")
+        if(NOT "${SRC_T}" STREQUAL "" AND NOT "${OUT_T}" STREQUAL "")
+            if(SRC_T LESS_EQUAL OUT_T)
+                return()
+            endif()
+        endif()
+    endif()
+    message(STATUS "glslc (${STAGE}) ${SOURCE} -> ${OUTPUT}")
+    execute_process(
+        COMMAND "${GLSLC}" "-fshader-stage=${STAGE}" "${SOURCE}" -o "${OUTPUT}"
+        RESULT_VARIABLE RC
+        OUTPUT_VARIABLE STDOUT
+        ERROR_VARIABLE STDERR
+    )
+    if(NOT RC EQUAL 0)
+        message(FATAL_ERROR "glslc failed for ${SOURCE}:\n${STDERR}")
+    endif()
+endfunction()
+
+set(VERT_SOURCE "${SHADERS_DIR}/_common/passthrough.vk.glsl")
+set(VERT_OUTPUT "${SHADERS_DIR}/_common/passthrough.vert.spv")
+if(EXISTS "${VERT_SOURCE}")
+    _compile_to_spv("${VERT_SOURCE}" "vert" "${VERT_OUTPUT}")
+else()
+    message(WARNING "Missing shared vertex shader: ${VERT_SOURCE}")
+endif()
+
+file(GLOB SHADER_DIRS LIST_DIRECTORIES true RELATIVE "${SHADERS_DIR}" "${SHADERS_DIR}/*")
+foreach(DIR ${SHADER_DIRS})
+    set(ABS_DIR "${SHADERS_DIR}/${DIR}")
+    if(NOT IS_DIRECTORY "${ABS_DIR}")
+        continue()
+    endif()
+    string(SUBSTRING "${DIR}" 0 1 FIRST_CHAR)
+    if(FIRST_CHAR STREQUAL "_" OR FIRST_CHAR STREQUAL ".")
+        continue()
+    endif()
+
+    set(VK_SOURCE "${ABS_DIR}/${DIR}.vk.glsl")
+    set(SPV_OUTPUT "${ABS_DIR}/${DIR}.frag.spv")
+
+    if(EXISTS "${VK_SOURCE}")
+        _compile_to_spv("${VK_SOURCE}" "frag" "${SPV_OUTPUT}")
+    endif()
+endforeach()