Fix for Raspberry Pi 4 build

[srgarg]

While DearPyGui builds on Raspberry Pi 4 just fine, trying to run an executable will throw Glfw Error 65543: GLX: Failed to create context as mentioned in Issue #580. I believe the issue is that Rpi4 VideoCore6 GPU only supports either OpenGL 2 or OpenGL ES. Currently DearPyGui is creating a OpenGL 3 context. Therefore, slight DearPyGui source modification is needed to use the OpenGL ES context. (ocornut's imgui example_glfw_opengl3 is used as reference to make the changes below)

First steps. use rpi-update to obtain RPi specific VideoCore libs. This downloads the libGLESv2 needed below.
More info: https://www.raspberrypi.com/documentation/computers/os.html#rpi-update

Modify the following files in src folder as below:

mvLinuxSpecifics.h

#if defined(IMGUI_IMPL_OPENGL_ES2)
#include <GLES2/gl2.h>
#endif
// must add before
#include <GL/gl3w.h>
#include <GLFW/glfw3.h>

mvGraphics_linux.cpp, Line 12

const char* glsl_version = "#version 100"; // 130->100

mvViewport_linux.cpp, Line 151

const char* glsl_version = "#version 100"; // 130 -> 100
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2); // 3->2
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
glfwWindowHint(GLFW_CLIENT_API, GLFW_OPENGL_ES_API); // new

mvUtilities_linux.cpp, not sure if this change is needed, but I did it this way

//#include <GL/gl3w.h>
//#include <GLFW/glfw3.h>

distribution.cmake & embedded.cmake

# make these additions
else() # Linux
    add_definitions(-DIMGUI_IMPL_OPENGL_ES2) # added
...
    target_link_libraries(coreemb PRIVATE "-lcrypt -lpthread -ldl -lutil -lm" GL GLESv2 glfw python3.9d) # added GLESv2
endif()

$python3 setup.py install to build and install DearPyGui on Raspberry Pi 4. You should have working executables after this!

Note: Despite this fix, texture loading does not work. I think it is because of GL3W use. GL3W is utility lib compatible with OpenGL 3 only. Significant changes will need to be made to source beyond my ability for this

Note: Did not test on Rpi 3 and below

Hope this helps someone