2D camera immediate GUI avoid flipping textures

[deuxcents]

Hello,

On 2D camera you have decided to set the origin at the top left of the screen which is fine (screen coordinates). Some comment in the code would be nice: set a screen coordinate system in pixel - i will open a separate token regarding dpi handling in windows.
Can you please store in gs_immediate_draw_t the fact that y axis is reversed and passing that info to the fragment shader in a new uniform ? That way the user does not have to do it on his side before hand. See pull request #31 for this proposed implementation.

Also:
GS_API_DECL void gsi_rect(gs_immediate_draw_t* gsi, float x0, float y0, float x1, float y1, uint8_t r, uint8_t g, uint8_t b, uint8_t a, gs_graphics_primitive_type type);

Should be (wrong variable names):
GS_API_DECL void gsi_rect(gs_immediate_draw_t* gsi, float l, float b, float r, float t, uint8_t r, uint8_t g, uint8_t b, uint8_t a, gs_graphics_primitive_type type);

Thanks !