Textures with alpha format default to SDL_BLENDMODE_BLEND

Fixes https://github.com/libsdl-org/SDL/issues/9941

docs/README-migration.md

@@ -1265,6 +1265,8 @@ SDL_CreateWindowAndRenderer() now takes the window title as the first parameter.
 
 SDL_GetRendererInfo() has been removed. The name of a renderer can be retrieved using SDL_GetRendererName(), and the other information is available as properties on the renderer.
 
+Textures are created with SDL_SCALEMODE_LINEAR by default, and use SDL_BLENDMODE_BLEND by default if they are created with a format that has an alpha channel.
+
 SDL_QueryTexture() has been removed. The properties of the texture can be queried using SDL_PROP_TEXTURE_FORMAT_NUMBER, SDL_PROP_TEXTURE_ACCESS_NUMBER, SDL_PROP_TEXTURE_WIDTH_NUMBER, and SDL_PROP_TEXTURE_HEIGHT_NUMBER. A function SDL_GetTextureSize() has been added to get the size of the texture as floating point values.
 
 Mouse and touch events are no longer filtered to change their coordinates, instead you

src/render/SDL_render.c

@@ -1355,6 +1355,7 @@ SDL_Texture *SDL_CreateTextureWithProperties(SDL_Renderer *renderer, SDL_Propert
     texture->color.g = 1.0f;
     texture->color.b = 1.0f;
     texture->color.a = 1.0f;
+    texture->blendMode = SDL_ISPIXELFORMAT_ALPHA(format) ? SDL_BLENDMODE_BLEND : SDL_BLENDMODE_NONE;
     texture->scaleMode = SDL_SCALEMODE_LINEAR;
     texture->view.pixel_w = w;
     texture->view.pixel_h = h;

test/testautomation_render.c

@@ -401,6 +401,12 @@ static void testBlendModeOperation(TestRenderOperation op, int mode, SDL_PixelFo
     if (dst == NULL) {
         return;
     }
+    if (SDL_ISPIXELFORMAT_ALPHA(dst_format)) {
+        SDL_BlendMode blendMode = SDL_BLENDMODE_NONE;
+        ret = SDL_GetTextureBlendMode(dst, &blendMode);
+        SDLTest_AssertCheck(ret == 0, "Verify result from SDL_GetTextureBlendMode(), expected: 0, got: %i", ret);
+        SDLTest_AssertCheck(blendMode == SDL_BLENDMODE_BLEND, "Verify alpha texture blend mode, expected %d, got %" SDL_PRIu32, SDL_BLENDMODE_BLEND, blendMode);
+    }
 
     /* Set as render target */
     SDL_SetRenderTarget(renderer, dst);