[Renderers/SDL3] Adds an example using SDL3 as a renderer (#107)

Co-authored-by: Nic Barker <contact+github@nicbarker.com>

renderers/SDL3/README

@@ -0,0 +1,6 @@
+Please note, the SDL3 renderer is not 100% feature complete. It is currently missing:
+
+- Rounded rectangle corners
+- Borders
+- Images
+- Scroll / Scissor handling

renderers/SDL3/clay_renderer_SDL3.c

@@ -0,0 +1,41 @@
+#include "../../clay.h"
+#include <SDL3/SDL_main.h>
+#include <SDL3/SDL.h>
+#include <SDL3_ttf/SDL_ttf.h>
+
+/* This needs to be global because the "MeasureText" callback doesn't have a
+ * user data parameter */
+static TTF_Font *gFonts[1];
+
+static void SDL_RenderClayCommands(SDL_Renderer *renderer, Clay_RenderCommandArray *rcommands)
+{
+    for (size_t i = 0; i < rcommands->length; i++) {
+        Clay_RenderCommand *rcmd = Clay_RenderCommandArray_Get(rcommands, i);
+        Clay_BoundingBox bounding_box = rcmd->boundingBox;
+        const SDL_FRect rect = { bounding_box.x, bounding_box.y, bounding_box.width, bounding_box.height };
+
+        switch (rcmd->commandType) {
+            case CLAY_RENDER_COMMAND_TYPE_RECTANGLE: {
+                Clay_RectangleElementConfig *config = rcmd->config.rectangleElementConfig;
+                Clay_Color color = config->color;
+                SDL_SetRenderDrawColor(renderer, color.r, color.g, color.b, color.a);
+                SDL_RenderFillRect(renderer, &rect);
+            } break;
+            case CLAY_RENDER_COMMAND_TYPE_TEXT: {
+                Clay_TextElementConfig *config = rcmd->config.textElementConfig;
+                Clay_String *text = &rcmd->text;
+                SDL_Color color = { config->textColor.r, config->textColor.g, config->textColor.b, config->textColor.a };
+
+                TTF_Font *font = gFonts[config->fontId];
+                SDL_Surface *surface = TTF_RenderText_Blended(font, text->chars, text->length, color);
+                SDL_Texture *texture = SDL_CreateTextureFromSurface(renderer, surface);
+                SDL_RenderTexture(renderer, texture, NULL, &rect);
+
+                SDL_DestroySurface(surface);
+                SDL_DestroyTexture(texture);
+            } break;
+            default:
+                SDL_Log("Unknown render command type: %d", rcmd->commandType);
+        }
+    }
+}