Fix blend func state save/restore

AFAIK there's no such enum as `GL_BLEND_SRC` or `GL_BLEND_DST` in OpenGL3 and OpenGL4.

Maybe they're left over from previous versions? They don't even seem to exist in OpenGL 2.1. In fact checking they were deprecated in OpenGL 1.5 (they exist in 1.3)

examples/opengl3_example/imgui_impl_glfw_gl3.cpp

@@ -51,12 +51,14 @@ void ImGui_ImplGlfwGL3_RenderDrawLists(ImDrawData* draw_data)
     GLint last_array_buffer; glGetIntegerv(GL_ARRAY_BUFFER_BINDING, &last_array_buffer);
     GLint last_element_array_buffer; glGetIntegerv(GL_ELEMENT_ARRAY_BUFFER_BINDING, &last_element_array_buffer);
     GLint last_vertex_array; glGetIntegerv(GL_VERTEX_ARRAY_BINDING, &last_vertex_array);
-    GLint last_blend_src; glGetIntegerv(GL_BLEND_SRC, &last_blend_src);
-    GLint last_blend_dst; glGetIntegerv(GL_BLEND_DST, &last_blend_dst);
+    GLint last_blend_src_rgb; glGetIntegerv(GL_BLEND_SRC_RGB, &last_blend_src_rgb);
+    GLint last_blend_dst_rgb; glGetIntegerv(GL_BLEND_DST_RGB, &last_blend_dst_rgb);
+    GLint last_blend_src_alpha; glGetIntegerv(GL_BLEND_SRC_ALPHA, &last_blend_src_alpha);
+    GLint last_blend_dst_alpha; glGetIntegerv(GL_BLEND_DST_ALPHA, &last_blend_dst_alpha);
     GLint last_blend_equation_rgb; glGetIntegerv(GL_BLEND_EQUATION_RGB, &last_blend_equation_rgb);
     GLint last_blend_equation_alpha; glGetIntegerv(GL_BLEND_EQUATION_ALPHA, &last_blend_equation_alpha);
     GLint last_viewport[4]; glGetIntegerv(GL_VIEWPORT, last_viewport);
-    GLint last_scissor_box[4]; glGetIntegerv(GL_SCISSOR_BOX, last_scissor_box); 
+    GLint last_scissor_box[4]; glGetIntegerv(GL_SCISSOR_BOX, last_scissor_box);
     GLboolean last_enable_blend = glIsEnabled(GL_BLEND);
     GLboolean last_enable_cull_face = glIsEnabled(GL_CULL_FACE);
     GLboolean last_enable_depth_test = glIsEnabled(GL_DEPTH_TEST);
@@ -120,7 +122,7 @@ void ImGui_ImplGlfwGL3_RenderDrawLists(ImDrawData* draw_data)
     glBindBuffer(GL_ARRAY_BUFFER, last_array_buffer);
     glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, last_element_array_buffer);
     glBlendEquationSeparate(last_blend_equation_rgb, last_blend_equation_alpha);
-    glBlendFunc(last_blend_src, last_blend_dst);
+    glBlendFuncSeparate(last_blend_src_rgb, last_blend_dst_rgb, last_blend_src_alpha, last_blend_dst_alpha);
     if (last_enable_blend) glEnable(GL_BLEND); else glDisable(GL_BLEND);
     if (last_enable_cull_face) glEnable(GL_CULL_FACE); else glDisable(GL_CULL_FACE);
     if (last_enable_depth_test) glEnable(GL_DEPTH_TEST); else glDisable(GL_DEPTH_TEST);