style: unify identifier naming per updated conventions

Automated with clang-tidy readability-identifier-naming (config added to
.clang-tidy) plus scripted passes, per the updated rules now documented
in CONTRIBUTING.md:

- types (class/struct/enum/alias/template params): PascalCase
- functions, variables, members: snake_case (incl. rational -> Rational)
- private/protected members: trailing underscore; static member
  variables likewise (instance_, available_themes_)
- constants and enum values: snake_case (kLinear -> k_linear,
  F32P -> f32p); ALL_CAPS reserved for macros
- macros: OAK_ prefix (OLIVE_ADD_TEST/OLIVE_ASSERT/OLIVE_CONFIG ->
  OAK_ADD_TEST/OAK_ASSERT/OAK_CONFIG, GL_PREAMBLE -> OAK_GL_PREAMBLE,
  include guards -> OAK_*)
- file names: all lowercase (Current/Plugin/OliveHost/OliveClip/
  OlivePluginInstance -> current/plugin/olivehost/oliveclip/
  oliveplugininstance)
- getters share the member name sans underscore, setters set_foo()
- Qt and third-party (OpenFX) virtual overrides and framework callbacks
  keep their original names (exempt in .clang-tidy)

Manual follow-ups required where automation could not reach:
- string-based QMetaObject/SIGNAL/SLOT references updated to renamed
  methods (AddTask, CreatedFile, DeleteSpecificFile, moveSelectionUp, ...)
- macro bodies referencing renamed methods (OLIVE_CONFIG,
  NODE_DEFAULT_DESTRUCTOR, MANAGEDDISPLAYWIDGET_*)
- self-shadowing locals renamed where signals/methods became same-named
  (size_changed, worker_count, selected_items, import param, filters)
- third_party OFX member/namespace usages restored (OFX::Host::*,
  _created, _clipPrefsDirty, createInstance, clearPersistentMessage)
- STL protocol aliases restored (const_iterator) with .clang-tidy
  ignore rules; qHash overloads restored

Full build and test suite pass: ctest 4/4, ~1960 gtest cases green.

app/render/backend/renderbackend_c.h

@@ -1,5 +1,5 @@
-#ifndef RENDERBACKEND_C_H
-#define RENDERBACKEND_C_H
+#ifndef OAK_RENDERBACKEND_C_H
+#define OAK_RENDERBACKEND_C_H
 
 #include <stdbool.h>
 #include <stdint.h>
@@ -20,20 +20,20 @@ typedef void *OakRenderBackendHandle;
 
 /* Identifies the concrete backend behind a dynamically loaded library. */
 enum OakRenderBackendKind {
-	OAK_RENDER_BACKEND_UNKNOWN = 0,
-	OAK_RENDER_BACKEND_OPENGL = 1,
-	OAK_RENDER_BACKEND_VULKAN = 2
+	oak_render_backend_unknown = 0,
+	oak_render_backend_opengl = 1,
+	oak_render_backend_vulkan = 2
 };
 
 /* Capability bits advertised by a backend through oak_renderer_get_info(). */
 enum OakRenderBackendCapability {
-	OAK_RENDER_BACKEND_CAP_TEXTURES = 1ULL << 0,
-	OAK_RENDER_BACKEND_CAP_SHADERS = 1ULL << 1,
-	OAK_RENDER_BACKEND_CAP_BLIT = 1ULL << 2,
-	OAK_RENDER_BACKEND_CAP_READBACK = 1ULL << 3,
-	OAK_RENDER_BACKEND_CAP_VIEWER_CONTEXT = 1ULL << 4,
-	OAK_RENDER_BACKEND_CAP_INSTANCE = 1ULL << 5,
-	OAK_RENDER_BACKEND_CAP_DEVICE = 1ULL << 6
+	oak_render_backend_cap_textures = 1ULL << 0,
+	oak_render_backend_cap_shaders = 1ULL << 1,
+	oak_render_backend_cap_blit = 1ULL << 2,
+	oak_render_backend_cap_readback = 1ULL << 3,
+	oak_render_backend_cap_viewer_context = 1ULL << 4,
+	oak_render_backend_cap_instance = 1ULL << 5,
+	oak_render_backend_cap_device = 1ULL << 6
 };
 
 /* Static and runtime metadata returned by the backend. */
@@ -118,4 +118,4 @@ typedef void *(*OakBackendOpenGLContextFn)(OakRenderBackendHandle handle);
 }
 #endif
 
-#endif // RENDERBACKEND_C_H
+#endif // OAK_RENDERBACKEND_C_H