egl: favor truecolor configurations.

If app requested <= 16 color depth and there is a 24-bit config available,
favor that. This fixes things that quietly expect to get truecolor output
but don't request it (...like SDL's render api...) and things that are
probably requesting 16-bit color as a fallback but expecting reasonable
systems to give them full depth.

Specifically, this fixes Life is Strange on Wayland, which uses the latter
approach, and anything using SDL_Render on Wayland, which uses the former.

Fixes #4056.
Fixes #4132.

src/video/SDL_egl.c

@@ -700,7 +700,8 @@ SDL_EGL_ChooseConfig(_THIS)
     /* 128 seems even nicer here */
     EGLConfig configs[128];
     SDL_bool has_matching_format = SDL_FALSE;
-    int i, j, best_bitdiff = -1, bitdiff;
+    int i, j, best_bitdiff = -1, best_truecolor_bitdiff = -1;
+    int truecolor_config_idx = -1;
    
     if (!_this->egl_data) {
         /* The EGL library wasn't loaded, SDL_GetError() should have info */
@@ -804,17 +805,30 @@ SDL_EGL_ChooseConfig(_THIS)
     /* From those, we select the one that matches our requirements more closely via a makeshift algorithm */
 
     for (i = 0; i < found_configs; i++ ) {
-        if (has_matching_format && _this->egl_data->egl_required_visual_id)
+        SDL_bool is_truecolor = SDL_FALSE;
-        {
+        int bitdiff = 0;
+
+        if (has_matching_format && _this->egl_data->egl_required_visual_id) {
             EGLint format;
             _this->egl_data->eglGetConfigAttrib(_this->egl_data->egl_display,
                                             configs[i], 
                                             EGL_NATIVE_VISUAL_ID, &format);
-            if (_this->egl_data->egl_required_visual_id != format)
+            if (_this->egl_data->egl_required_visual_id != format) {
                 continue;
+            }
+        }
+
+        _this->egl_data->eglGetConfigAttrib(_this->egl_data->egl_display, configs[i], EGL_RED_SIZE, &value);
+        if (value == 8) {
+            _this->egl_data->eglGetConfigAttrib(_this->egl_data->egl_display, configs[i], EGL_GREEN_SIZE, &value);
+            if (value == 8) {
+                _this->egl_data->eglGetConfigAttrib(_this->egl_data->egl_display, configs[i], EGL_BLUE_SIZE, &value);
+                if (value == 8) {
+                    is_truecolor = SDL_TRUE;
+                }
+            }
         }
 
-        bitdiff = 0;
         for (j = 0; j < SDL_arraysize(attribs) - 1; j += 2) {
             if (attribs[j] == EGL_NONE) {
                break;
@@ -832,17 +846,38 @@ SDL_EGL_ChooseConfig(_THIS)
             }
         }
 
-        if (bitdiff < best_bitdiff || best_bitdiff == -1) {
+        if ((bitdiff < best_bitdiff) || (best_bitdiff == -1)) {
             _this->egl_data->egl_config = configs[i];
-            
             best_bitdiff = bitdiff;
         }
 
-        if (bitdiff == 0) {
+        if (is_truecolor && ((bitdiff < best_truecolor_bitdiff) || (best_truecolor_bitdiff == -1))) {
-            break; /* we found an exact match! */
+            truecolor_config_idx = i;
+            best_truecolor_bitdiff = bitdiff;
         }
     }
 
+    #define FAVOR_TRUECOLOR 1
+    #if FAVOR_TRUECOLOR
+    /* Some apps request a low color depth, either because they _assume_
+       they'll get a larger one but don't want to fail if only smaller ones
+       are available, or they just never called SDL_GL_SetAttribute at all and
+       got a tiny default. For these cases, a game that would otherwise run
+       at 24-bit color might get dithered down to something smaller, which is
+       worth avoiding. If the app requested <= 16 bit color and an exact 24-bit
+       match is available, favor that. Otherwise, we look for the closest
+       match. Note that while the API promises what you request _or better_,
+       it's feasible this can be disastrous for performance for custom software
+       on small hardware that all expected to actually get 16-bit color. In this
+       case, turn off FAVOR_TRUECOLOR (and maybe send a patch to make this more
+       flexible). */
+    if ( ((_this->gl_config.red_size + _this->gl_config.blue_size + _this->gl_config.green_size) <= 16) ) {
+        if (truecolor_config_idx != -1) {
+            _this->egl_data->egl_config = configs[truecolor_config_idx];
+        }
+    }
+    #endif
+
 #ifdef DUMP_EGL_CONFIG
     dumpconfig(_this, _this->egl_data->egl_config);
 #endif